Optical recording method and optical recording medium

ABSTRACT

An optical recording method for recording mark length-modulated information on a recording medium by using a plurality of recording mark lengths. The optical recording method comprises the steps of:
         when a time length of one recording mark is denoted nT (T is a reference clock period equal to or less than 25 ns, and n is a natural number equal to or more than 2),
           (i) dividing the time length of the recording mark nT into
 
η 1 T, α 1 T, β 1 T, α 2 T, β 2 T, . . . , α i T, β i T, . . . , α m T, β m T, η 2 T
    in that order (m is a pulse division number, Σ i (α i +β i )+η 1 +η 2 =n; α i  (1≦i≦m) is a real number &gt;0; β i  (1≦i≦m−1) is a real number&gt;0; β m  is a real number≧0; and η 1  is a real number of −2≦η 1 ≦2 and η 2  is a real number of −2≦η 2 ≦2);   
           radiating recording light with a recording power Pw i  in a time duration of α i T (1≦i≦m), and radiating recording light with a bias power Pb i  in a time duration of β i T (1≦i≦m), the bias power being Pb i &lt;Pw i  and Pb i &lt;Pw i+1 ; and   (ii) changing m, α i , β i , η 1 , η 2 , Pw i  and Pb i  according to n of the time length nT of the recording mark;
 
wherein the pulse division number m is 2 or more for the time duration of at least one recording mark and meets n/m≧1.25 for the time length of all the recording marks.

This is a division of application Ser. No. 09/884,121 filed Jun. 20,2001, now U.S. Pat. No. 6,411,579, which is a continuation applicationof International patent application No.PCT/JP00/03036, filed May 11,2000.

TECHNICAL FIELD

The present invention relates to an optical recording method and anoptical recording medium.

BACKGROUND ART

As the amount of information increases in recent years, there aregrowing demands for a recording medium capable of writing and retrievinga large amount of data at high speed and in high density. There aregrowing expectations that the optical disks will meet this demand.

There are two types of optical disks: a write-once type that allows theuser to record data only once, and a rewritable type that allows theuser to record and erase data as many times as they wish. Examples ofthe rewritable optical disk include a magnetooptical recording mediumthat utilizes a magneto-optical effect and a phase-change type recordingmedium that utilizes a change in reflectance accompanying a reversiblecrystal state change.

The principle of recording an optical disk involves applying a recordingpower to a recording layer to raise the temperature of that layer to orabove a predetermined critical temperature to cause a physical orchemical change for data recording. This principle applies to all of thefollowing media: a write-once medium utilizing pitting or deformation,an magnetooptical medium utilizing a magnetic reversal at the vicinityof the Curie point, and a phase change medium utilizing a phasetransition between amorphous and crystal states of the recording layer.

Further, taking advantage of the 1-beam-overwrite ability (erasing andwriting at the same time) of the phase change recording medium,rewritable compact disks compatible with CDs and DVDs (CD-ReWritable andCD-RW) and rewritable DVDs have been developed.

Almost all of these optical recording media in recent years employ amark length recording method, which is suited for increasing therecording density.

The mark length recording is a method that records data by changing boththe lengths of marks and the lengths of spaces. Compared with a markposition recording method which changes only the lengths of the spaces,this method is more suited to increasing the recording density and canincrease the recording density by as much as 1.5 times. However, toretrieve data accurately makes the detection of the time length of themark stringent, thus requiring precise control of the shape of markedges. Further, there is another difficulty that a plurality of kinds ofmarks with different lengths, from short marks to long marks, need to beformed.

In the following descriptions, the spatial length of a mark is referredto as a mark length and a time length of the mark as a mark time length.When a reference clock period is determined, the mark length and themark time length have a one-to-one correspondence.

In the mark length recording. when writing an nT mark (a mark having amark time length of nT where T is a reference clock period of data and nis a natural number), simply radiating a recording power of square wavewith the time length of nT or with the length finely adjusted willresult in the front and rear ends of each mark differing in temperaturedistribution, which in turn causes the rear end portion in particular toaccumulate heat and widen, forming an mark with an asymmetric geometry.This raises difficulties in precisely controlling the mark length andsuppressing variations of the mark edge.

To uniformly shape the marks, from short marks to long marks, variousmeans have been employed, such as division of recording pulses and useof off pulses. For example, the following techniques have been adoptedin the phase change media.

That is, a recording pulse is divided to adjust the geometry of anamorphous mark (JP-A62-259229, JP-A63-266632). This approach is alsoutilized in the write-once medium that is not overwritten. Further, anoff pulse is widely employed as a mark shape compensation means (JP-A63-22439, etc.)

Other proposed methods include one which deliberately dull a trailingedge of the recording pulse to adjust the mark length and the mark timelength (JP-A 7-37252); one which shifts a recording pulse radiation time(JP-A 8-287465); one which, in a multipulse recording method,differentiates a value of bias power during the mark writing operationfrom that during the space writing operation or erasing operation (JP-A7-37251); and one which controls a cooling time according to a linearvelocity (JP-A 9-7176).

The recording method based on the above pulse division approach is alsoused in the magnetooptical recording medium and the write-once typeoptical recording medium. In the magnetooptical and write-once typemediums, this approach aims to prevent heat from becoming localized. Inthe phase change medium, this approach has additional objective ofpreventing recrystallization.

Common examples of mark length modulation recording include a CDcompatible medium using an EFM (Eight-Fourteen Modulation), a DVDcompatible medium using an EFM+ modulation, a variation of 8-16modulation, and a magnetooptical recording medium using a (1,7)-RLL-NRZI (Ruu-Length Limited Non-Return to Zero Inverted) modulation.The EFM modulation provides 3T to 11T marks; the EFM+ modulationprovides 3T to 14T marks; and the (1, 7)-RLL-NRZI modulation provides 2Tto 8T marks. Of these, the EFM+ modulation and the (1, 7)-RLL-NRZImodulation are known as modulation methods for high-density mark lengthmodulation recording.

As the recording pulse division scheme for the mark length modulationrecording media such as CD, the following method is widely used.

That is, when a mark to be recorded has a time length of nT (T is areference clock period and n is a natural number equal to or greaterthan 2), the time (n−η)T is divided intoα₁T, β₁T, α₂T, β₂T, . . . , α_(m)T, β_(m)T(where Σα_(i)+Σβ_(i)=n−η; η is a real number from 0 to 2; m is a numbersatisfying m=m−k; and k is 1 or 2). In a time duration of α_(i)T (1≦i≦m)as the recording pulse section, recording light with a recording powerPw is radiated. In a time duration of β_(i)T (1≦i≦m) as the off pulsesection, recording light with a bias power Pb, less than Pw, isradiated.

FIG. 2 is a schematic diagram showing a power pattern of the recordinglight used in this recording method. To form a mark of a length shown inFIG. 2(a), a pattern shown in FIG. 2(b) is used. When forming a markthat is mark-length-modulated to the length of nT (T is a referenceclock period; and n is a mark length, an integer value, that can betaken in the mark length modulation recording), (n−η)T is divided intom=n−k (k is 1 or 2) recording pulses (in the case of FIG. 2(b), k=1 andη=0.5), and the individual recording pulse widths are set to α_(i)T(1≦i≦m), each followed by the off pulse section of β_(i)T (1≦i≦m). Inthe α_(i)T (1≦i≦m) section during the recording, the recording lightwith the recording power Pw is radiated and, in the β_(i)T (1≦i≦m)section, the bias power Pb (Pb<Pw) is radiated. At this time, to ensurethat an accurate nT mark can be obtained during the detection of themark length, Σα_(i)+Σβ_(i) may be set slightly smaller than n, and thefollowing setting is made: Σα_(i)+Σβ_(i)=n−η (η is a real number in0.0≦η≦2.0).

That is, in the conventional technique, when the recording light to beradiated to form an nT mark is divided, the recording pulse is dividedinto m pieces (m=n−k, where k is 1 or 2), m being obtained by uniformlysubtracting k from n (as described in JP-A 9-282661), and then apredetermined number is subtracted from the number of divisions m of therecording pulse to control the mark time length accurately (in thefollowing, such a pulse division scheme is called an “n−k division”scheme).

Generally, the reference clock period T decreases as the density orspeed increases. For example, T decreases in the following cases.

(1) When the Recording Density is Enhanced to Increase the RecordingCapacity

As the mark length and the mark time length are reduced, the densityincreases. In this case, a clock frequency needs to be increased toreduce the reference clock period T.

(2) When the Recording Linear Velocity is Increased to Increase a DataTransfer Rate

In the high-speed recording of recordable CDs and DVDs, the clockfrequency is increased to reduce the reference clock period T. In aCD-based medium such as a rewritable compact disk, for example, thereference clock period T during a ×1-speed operation (linear velocity is1.2-1.4 m/s) is 231 nanoseconds; but during a ×10-speed operation thereference clock period T becomes very short, 23.1 nanoseconds. In theDVD-based medium, while the reference clock frequency T during a×1-speed operation (3.5 m/s) is 38.2 nanoseconds, it is 19.1 ns during a×2-speed operation.

As can be seen from the (1) and (2), in large-capacity optical disks andCDs and DVDs with high data transfer rates, the reference clock period Tis very short. As a result, the recording pulse section α_(i)T and theoff pulse section β_(i)T also tend to become short. Under thesecircumstances the following problems arise.

Problem a

The recording pulse section α_(i)T may be too short for therising/falling edge speed of radiated light, particularly a laser, tofollow. A rise time is a time taken by the projected power of radiatedlight such as laser to reach a set value, and a fall time is a timetaken by the projected power of the radiated light such as laser to fallfrom the set value to a complete off level. At present the rise and falltimes take at least 2-3 nanoseconds respectively. Hence, when the pulsewidth is less than 15 ns. for example, the time it takes for the lightto actually project a required power is a few nanoseconds. Further, whenthe pulse width is less than five nanoseconds, the projected powerbegins to fall before it reaches the set value, so that the temperatureof the recording layer does not rise sufficiently, failing to produce apredetermined mark size. These issues of response speed limits of asignal source and a laser beam cannot be dealt with by makingimprovements on the wavelength of a light source, on the method ofradiating light onto substrate/film surface, or on other recordingmethods.

Problem b

When the off pulse section β_(i)T is narrow, the recording medium cannottake a sufficient time to cool down and the off pulse function (coolingspeed control function) does not work although the off pulse section isprovided, leaving heat to be accumulated in the rear end part of themark, making it impossible to form the correct shape of the mark. Thisproblem becomes more serious as the length of the mark increases.

This problem will be explained by taking a phase change medium as anexample.

The currently available phase change medium typically takes crystalportions as an unrecorded state or erased state and amorphous portionsas a recorded state. To form an amorphous mark involves radiating alaser onto a tiny area of the recording layer to melt that tiny portionand quickly cooling it to form an amorphous mark. When, for example, along mark (a mark more than about 5T in length based on the EFMmodulation recording for CD format) is formed using a rectangularwaveform of recording power with no off pulse section at all, as shownin FIG. 3(a), then an amorphous mark with a narrow rear end is formed asshown in FIG. 3(b) and a distorted retrieve waveform is observed asshown in FIG. 3(c). This is because, in the rear part of the long markin particular, heat is accumulated by heat diffusion from the front partenlarging the melted area in the rear part but the cooling speeddeteriorates significantly allowing the melted area to recrystallize asit solidifies. This tendency becomes conspicuous as the linear velocityfor recording decreases because the cooling speed of the recording layerbecomes slower as the linear velocity decreases.

Conversely, if the cooling speed is so high as to renderrecrystallization almost negligible, when a long mark is recorded, anamorphous mark with a thicker rear end is formed as shown in FIG. 3(d),producing a distorted retrieve waveform as shown in FIG. 3(e). This isexplained as follows. In the rear end of the long mark in particular,heat is accumulated by heat diffusion from the front part enlarging themelted area in the rear part and the shape of the melted area istransformed into the shape of an amorphous mark relatively preciselybecause the cooling speed is kept relatively high over the entire area.

When a plurality of off pulse sections are not distributed and properlyused over the entire mark length, recrystallization becomes conspicuoussomewhere in the mark, as shown in FIGS. 3(b) and 3(d) though indifferent degrees, preventing a good formation of an amorphous long markand causing distortions in the retrieve waveform.

Inserting the off pulse sections makes sharp the temperature change overtime of the recording layer ranging from the front end to the rear endof the long mark, preventing degradation of the mark due torecrystallization during recording.

However, as the reference clock frequency T becomes shorter because ofincreased density and speed as described above, the rapid coolingbecomes difficult to achieve even with the off pulse sections providedin a conventional manner, resulting in the front half of the mark beingrecrystallized.

For example, when a mark with a time length of 4T is to be recorded on aCD-RW, a phase change type rewritable compact disk, by the conventionaln−k division scheme (k=1), the following pulses are radiated during theprocess of forming the amorphous mark:α₁T, β₁T, α₂T, β₂T, α₃T, β₃T

Here, the starting end of the mark is melted by the application of therecording pulse α₁T and then heat produced by the application of thesubsequent recording pulses α₂T, α₃T conducts toward the front part ofthe mark. FIG. 4 is a schematic temperature history of the mark startingend, with FIG. 4(a) representing a case in which the linear velocity islow and FIG. 4(b) a case in which the linear velocity is high. In eithercase, three temperature rising processes due to α₁T, α₂T, α₃T and threecooling processes due to β₁T, β₂T, β₃T are observed.

In the case of low linear velocity, as shown in FIG. 4(a), there aresufficient cooling times at β₁T, β₂T, during each of which thetemperature of the cooling layer can fall below the crystallizationtemperature. In the case of high linear velocity, however, because thereference clock period T decreases in inverse proportion to the linearvelocity, the recording layer melted by the α₁T is heated by the nextα₂T and further by α₃T without cooling below the crystallizationtemperature range, as shown in FIG. 4(b). The time during which therecording layer stays in the crystallization temperature range is muchlonger for T₄+T₅+T₆ of the high linear velocity than for T₁+T₂+T₃ of thelow linear velocity, so it is understood that the recrystallization ismore likely to take place at the fast linear velocity. In an alloy witha composition close to a SbTe eutectic composition and used as a phasechange recording layer, a crystal is likely to grow at theamorphous/crystal boundary and therefore recrystallization easily occursouter area of the mark. Here, the low speed refers to less than about×10-speed (T=less than 23.1 nanoseconds) and the high speed refers toabout ×10-speed or more.

As described above, in the phase change medium, as the reference clockperiod T becomes short due to an increased density and speed,recrystallization is likely to occur with the conventional pulsedivision scheme, giving rise to a serious problem that a required degreeof modulation fails to be generated at the central part of the longmark.

In the phase change medium in which an amorphous mark is recorded over acrystal area, although it is generally easy at high linear velocity tosecure an enough cooling speed to form an amorphous solid, thecrystallization time is difficult to secure. Hence, the phase changemedium often employs a recording layer of a composition which tends tobe easily crystallized, i.e., a recording layer of an easilyrecrystallizable composition. Therefore, it is important to increase theoff pulse section to enhance the cooling effect, but during the highlinear velocity the off pulse section becomes short to the contrary.

The similar problem is also encountered when the wavelength of a lasersource is reduced or a numerical aperture is increased to reduce a beamdiameter for enhancing the density of the phase change medium. Forexample, when a laser with a wavelength of 780 nm and a numericalaperture of NA=50 is changed to a laser with a wavelength of 400 nm anda numerical aperture of 0.65, the beam diameter is throttled to almostone-half. At this time, the energy distribution in the beam becomessteep so that the heated portion is easily cooled, allowing an amorphousmark to be formed easily. This however makes the recording layer moredifficult to crystallize. In this case, too, it is necessary to increasethe cooling effect.

The present invention has been accomplished to solve the aforementionedproblems. It is an object of the invention to provide an opticalrecording method and an optical recording medium suited for the method,which can perform recording in a satisfactory manner even during a marklength recording using a short clock period suited for high densityrecording and high speed recording.

DISCLOSURE OF THE INVENTION

The inventors of this invention have found that the above objective canbe realized by reducing the number of divisions m in the pulse divisionscheme from the conventional division number.

Viewed from one aspect the present invention provides an opticalrecording method for recording mark length-modulated information with aplurality of recording mark lengths by radiating light against arecording medium, the optical recording method comprising the steps of:

-   -   when a time length of one recording mark is denoted nT (T is a        reference clock period equal to or less than 25 ns, and n is a        natural number equal to or more than 2),    -   dividing the time length of the recording mark nT into        η₁T, α₁T, β₁T, α₂T, β₂T, . . . , α_(i)T, β_(i)T, . . . , α_(m)T,        β_(m)T, η₂T    -    in that order (m is a pulse division number;        Σ_(i)(α_(i)+β_(i))+η₁+η₂=n; α_(i) (1≦i≦m) is a real number        larger than 0; β_(i) (1≦i≦m−1) is a real number larger than 0;        β_(m) is a real number larger than or equal to 0; and η₁ and η₂        are real numbers between −2 and 2); and    -   radiating recording light with a recording power Pw_(i) in a        time duration of α₁T (1≦i≦m), and radiating recording light with        a bias power Pb_(i) in a time duration of β_(i)T (1≧i ≧m−1), the        bias power being Pb_(i)<Pw_(i) and Pb_(i)<Pw_(i+1);    -   wherein the pulse division number m is 2 or more for the time        duration of at least one recording mark and meets n/m≧1.25 for        the time length of all the recording marks.

Viewed from another aspect, the present invention provides a phasechange type optical recording medium recorded by the optical recordingmethod, the phase change type optical recording medium having arecording layer made of M_(z)Ge_(y)(Sb_(x)Te_(1-x))_(1-y-z) alloy (where0≦z≦0.1, 0<y≦0.3, 0.8≦x; and M is at least one of In, Ga, Si, Sn, Pb,Pd, Pt, Zn, Au, Ag, Zr, Hf, V, Nb, Ta, Cr, Co, Mo, Mn, Bi, O, N and S).

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a)-(c) are explanatory diagrams showing an example recordingpulse division scheme and an example method of generating the recordingpulses according to the invention.

FIGS. 2(a)-(b) are explanatory diagrams showing a conventional recordingpulse division scheme.

FIGS. 3(a)-(e) are schematic diagrams showing a shape of a recorded markand a change of reflectance in a phase change optical recording medium.

FIGS. 4(a)-(b) are examples of temperature history when recording lightis radiated against the recording layer of the phase change opticalrecording medium.

FIG. 5 is a schematic diagram of retrieved waveforms (eye-pattern) of anEFM modulation signal.

FIGS. 6(a)-(c) are examples of division scheme of a recording pulse foran 11T mark according to an embodiment of the invention.

FIG. 7 is a graph showing a relation between α₁ and a mark time lengthin the embodiment 1 of the invention.

FIG. 8 is a graph showing a relation between β_(m) and a mark timelength in the embodiment 1 of the invention.

FIG. 9 is an example of division scheme of a recording pulse for an EFMrandom pattern in the embodiment 1 of the invention.

FIG. 10 is a graph showing a relation of measured values of mark timelength/space time length with respect to theoretical values in theembodiment 1 of the invention.

FIGS. 11(a)-(b) are examples of conventional division scheme of arecording pulse for a 11T mark/11T space.

FIGS. 12(a)-(c) are explanatory diagrams showing an example of a pulsedivision scheme according to the invention.

FIGS. 13(a)-(d) are explanatory diagrams showing a timing for generatinga gate in the pulse division scheme of FIG. 12.

FIGS. 14(a)-(b) are explanatory diagrams showing a pulse division schemein (1) of embodiment 3.

FIGS. 15(a)-(b) are graphs showing a dependency of a modulation in (1)of embodiment 3.

FIGS. 16(a)-(c) are explanatory diagrams showing a pulse division schemein (2) of embodiment 3.

FIGS. 17(a)-(b) are graphs showing a dependency of α₁ of a mark length(-▴-) and a space length (-◯-) in (2) of embodiment 3.

FIGS. 18(a)-(b) are graphs showing a dependency of β₁ of a mark length(-▴-) and a space length (-◯-) in (2) of embodiment 3.

FIGS. 19(a)-(b) are graphs showing a dependency of β_(m) of a marklength (-▴-) and a space length (-◯-) in (2) of embodiment 3.

FIG. 20 is an explanatory diagram showing a pulse division scheme in (3)of embodiment 3.

FIGS. 21(a)-(b) are graphs showing a mark length (-⋄-) and a spacelength (-●-), and their jitters in (3) of embodiment 3.

FIG. 22 is an explanatory diagram showing a pulse division scheme in (4)of embodiment 3.

FIGS. 23(a)-(b) are graphs showing a mark length (-⋄-) and a spacelength (-●-), and their jitters in (4) of embodiment 3.

FIGS. 24(a)-(c) are explanatory diagrams showing an example of a pulsedivision scheme according to the invention.

FIGS. 25(a)-(c) are explanatory diagrams showing an example of a pulsedivision scheme according to embodiment 4 and a dependency on Tw/T of amodulation obtained.

FIG. 26 is an explanatory diagram showing an example of a pulse divisionscheme according to embodiment 4 of the invention.

FIGS. 27(a)-(c) are diagrams showing a dependency on power of modulationand jitter and a dependency of jitter on the number of overwrites.

FIG. 28 is an explanatory diagram showing another example of a pulsedivision scheme according to embodiment 4.

PREFERRED EMBODIMENTS OF THE INVENTION

Now, the present invention will be described in detail by referring tothe accompanying drawings.

The optical recording method of this invention reduces the number ofdivisions in the pulse division scheme, i.e., elongates each pulse ofrecording light to make the time during which to heat a light-irradiatedportion of the optical recording medium sufficiently long with respectto the response speed of the laser pulse and also sets the time duringwhich to cool the light-irradiated portion sufficiently long. Thisenables satisfactory mark length recording even with a clock period aslow as 25 nm or less.

In more concrete terms, suppose the time length of a recording mark isnT (T is a reference clock period equal to or less than 25 ns; and n isa natural number equal to or more than 2). The time length nT of therecording mark is divided in the following order:η₁T, α₁T, β₁T, α₂T, β₂T, . . . , α_(i)T, β_(i)T, . . . , α_(m)T, β_(m)T,η₂T(m is a number of pulse divisions; Σ_(i)(α_(i)+β_(i))+η₁+η₂=n;α_(i)(1≦i≦m) is a real number larger than 0, β_(i)(1≦i≦m−1) is a realnumber larger than 0, and β_(m) is a real number equal to or larger than0; and η₁ and η₂ are real numbers equal to or larger than −2, preferably0, and equal to or smaller than 2, preferably 1). In the time length ofα_(i)T (1≦i≦m), recording light with a recording power Pw_(i) isradiated; and in the time length of β_(i)T (1≦i≦m), recording light witha bias power Pb_(i), which has the relation of Pb_(i)<Pw_(i) andPb_(i)<Pw_(i+1), is radiated. As for the time length of at least onerecording mark, the above pulse division number m is set to 2 or more;and as for the time length of all recording marks, n/m≧1.25.

That is, while the conventional n−k division scheme sets the pulsedivision number m equal to n−k (k is 1 or 2), this invention defines thepulse division number m from a different perspective.

In this invention, as to the time length of at least one recording markthe above pulse division number m is set to 2 or more. It should benoted, however, that there is no need to perform the pulse division forall nT marks (marks with a time length of nT; T is a reference clockperiod; and n is a natural number equal to or larger than 2). In shortmarks such as 2T, 3T and 4T, the problem of heat accumulation isrelatively small but the response speed of the pulse being unable tofollow the pulse division poses a more serious problem. It is thereforepreferred that only one pulse of recording light with a recording powerof Pw be radiated or that one pulse of recording light with therecording power of Pw and one pulse of recording light with a bias powerof Pb be radiated.

In this invention, as to the time lengths of all recording marks, it isassumed that n/m≧1.25.

Suppose that η₁ and η₂ are both 0. Then becauseΣ_(i)(α_(i)+β_(i))/m=n/m, the value of n/m corresponds to an averagelength of (α_(i)+β_(i)) and the value of (n/m)T corresponds to anaverage period of the divided pulse.

In the conventional n−k division scheme, m=n−k and k is fixed to 1 or 2,so that n/m=n/(n−1) or n/m=n/(n−2). This value decreases as n increases.Thus, if we let the longest mark time length be n_(max)T, then n/mbecomes minimum for n_(max). That is, because the average period of thedivided pulses is longest for the shortest mark and shortest for, thelongest mark, α_(i)T and β_(i)T are shortest for the longest mark.

For example, in the EFM modulation, n=3-11 and k=2, so(n_(max)/m)=11/(11-2)=about 1.22

Similarly, in the EFM+ modulation, n=3-14 and k=2, so(n_(max)/m)=14/(14-2)=about 1.16

In the (1, 7)-RLL-NRZI modulation, n=2-8 and k=1, so(n_(max)/m)=8/(8-1)=about 1.14

As can be seen from the above, in the conventional scheme the values ofn/m are approximately 1.22, 1.16 and 1.14. When the reference clockperiod T becomes shorter than about 25 nanoseconds. the average periodof the divided pulses in the longest mark is generally less than 25nanoseconds and the average value of the recording pulse section α_(i)Tor the average value of the off pulse section β_(i)T is less than 12.5nanoseconds. This means that for at least one i, either α_(i)T or β_(i)Tis less than 12.5 nanoseconds. Further, when the clock period T goesbelow approximately 20 seconds, either α_(i)T or β_(i)T becomes furthersmaller.

In the above explanation, if a particular α_(i) or β_(i) becomes longerthan the average, this means that other α_(i) or β_(i) becomes shorterand the fact still remains that either α_(i)T or β_(i)T becomes smaller.

To describe more accurately, in the n−k division scheme Σ(α_(i)+β_(i))is not necessarily equal to n and may be equal to n−η (η=0 to 2). Inthis case, the average value of α_(i) and β_(i) becomes further smaller,making the problem more serious.

In the optical recording method of this invention, m is set to satisfythe condition of n/m≧1.25 as to the time length of all recording marksranging from short to long marks. As a result, the lengths of α_(i)T andβ_(i)T are made sufficiently long. For example, the recording pulsesection α_(i)T and the off pulse section β_(i)T can generally be setslightly longer than 0.5T to sufficiently heat the recording layer andat the same time limit the heat being supplied from the subsequentpulses and thereby produce a sufficient cooling effect.

When a mark is long in particular, the shape of a mark is easilydeformed by the accumulated heat. Hence, for marks 7T or longer in timelength, n/m should preferably be set to 1.5 or more. It is of coursepreferred that, also for short marks 6T or shorter, n/m be set to 1.5 ormore, more preferably to 1.8 or more.

It is noted, however, that because too large a value of n/m increasesthe heat accumulation, normally n/m is preferably set to 4 or less, morepreferably 3 or less.

The optical recording method of this invention produces a greater effectas the reference clock period T decreases, and it is preferred that thereference clock period be set to 20 nm or less or more preferably 15 nsor less. A very short clock period is difficult to achieve in practiceand it is normally preferred that the clock period have 0.1 ns or more,or preferably 1 ns or more, or more preferably 3 ns or more. As theclock period T decreases, it is desired that the minimum value of n/m beincreased.

The recording mark in this invention is recognized as a physical markformed continuously in a recording medium and optically distinguishablefrom other portions. That is, the invention does not join, throughprocessing by a reproducing system, 2T, 3T and 4T marks of theconventional n−k division scheme that meet the condition of n/m≧1.25 andrecognize them as a single long mark. In this invention, however, therecording mark may be formed of a plurality of physical marks that arebelow the optical resolution power of the retrieveing light. If we letthe numerical aperture of an objective for focusing the retrieveinglight be NA and the wavelength of the retrieveing light be λ, when thephysical marks are spaced from each other by 0.2 (λ/NA) or more, thesephysical marks can be optically distinguishable as separate marks.Hence, when forming a recording mark using a plurality of physicalmarks, they should preferably be spaced within 0.2 (λ/NA) of each other.

In this invention, the parameters associated with the divided pulsessuch as α_(i), β_(i), η₁, and η₂, Pw and Pb can be changed as requiredaccording to the mark length and i.

Further, in this invention it is preferred that the average value of therecording pulse section α_(i)T (1≦i≦m) and the average value of the offpulse section β_(i)T (1≦i≦m−1) both be set to 3 nanoseconds or more,preferably 5 nanoseconds or more, or more preferably 10 nanoseconds ormore in terms of securing the response capability of the radiated light.More preferably, individual α_(i)T (1≦i≦m) and β_(i)T (1≦i≦m−1) are setto 3 nanoseconds or more, or 5 nanoseconds or more, or more specifically10 nanoseconds or more. The rise time and fall time of the power of thelaser beam normally used during the process of recording shouldpreferably be set 50% or less of the minimum α_(i)T (1≦i≦m) and β_(i)T(1≦i≦m).

In this invention, although it is possible to set β_(m) to 0 not toradiate light during the last off pulse section of β_(m)T, if the heataccumulation problem at the end of the mark is grave, β_(m)T shouldpreferably be provided. In that case, it is preferred that β_(m)T be setnormally to 3 nanoseconds or more, or specifically to 5 nanoseconds ormore, or more preferably to 10 nanoseconds or more.

When the recording pulse section α_(i)T (1≦i≦m) is three nanoseconds ormore, especially 5 nanoseconds or more, the radiation energy requiredfor recording can be secured by increasing the recording power Pw_(i)although there is a problem of the rising/falling edge of the recordinglight.

On the other hand, when the off pulse section β_(i)T (1≦i≦m−1), too, is3 nanoseconds or more, especially 5 nanoseconds or more, the coolingeffect can be secured by reducing the bias power Pb down to nearly theretrieveing light power Pr or to 0 as long as this is not detrimental toa tracking servo or others.

To obtain a still greater cooling effect, it is desired thatΣ_(i)(α_(i)) associated with the time length of all recording marks beset to 0.6 n or less, particularly 0.5 n or less. More preferably,Σ_(i)(α_(i)) is set to 0.4 n or less. That is, the sum of the recordingpulse sections Σ_(i)(α_(i)T) is set shorter than Σ_(i)(β_(i)T) so thatthe off pulse section in each mark is longer. It is particularlypreferred that, for all i of i=2 to m−1, α_(i)T≦β_(i)T, i.e., in therecording pulse train following at least a second pulse, β_(i)T is madelonger.

In the recording method of this invention, the values of α_(i) (1≦i≦m)and β_(i) (1≦i≦m−1) are set appropriately according to the values of therecording pulse section α_(i)T (1≦i≦m) and the off pulse section β_(i)T(1≦i≦m−1) and are normally set to 0.01 or more, preferably 0.05 or more,and normally 5 or less, preferably 3 or less. Too small a value of β_(i)(1≦i≦m−1) may result in an insufficient cooling effect and hence it ispreferably set to 0.5 or more, specifically 1 or more. On the otherhand, too large a value of β_(i) may cause an excessive cooling andresult in the recording mark being optically separated. Hence it ispreferably set to 2.5 or less, specifically 2 or less. The effect ofthis setting is particularly large in the first off pulse section β_(i)Tthat has a great effect on the shape of the front end of the mark.

What has been described above can also be said of the last off pulsesection β_(m)T that has a great effect on the shape of the rear end ofthe mark. Hence, β_(m) is normally set to 0.1 or more, preferably 0.5 ormore, more preferably 1 or more, and 2.5 or less, preferably 2 or less.The switching period of intermediate pulse sections (group) α_(i)T(2≦i≦m−1) between the start pulse section α₁T and the last pulse sectionα_(m)T should preferably be set constant in terms of simplifying thecircuit. In more concrete terms, (α_(i)+β_(i))T (2≦i≦m−1) or(α_(i)+β_(i−1))T (2≦i≦m−1) is preferably set to 1.5T, 2T or 2.5T.

In this invention, the recording light power Pb_(i) radiated during theoff pulse section β_(i)T (1≦i≦m−1) is set smaller than the powers Pw_(i)and Pw_(i+1) of the recording light radiated during the recording pulsesections α_(i)T and α_(i+1)T. To obtain a large cooling effect, it ispreferred that Pb_(i)<Pw_(i) be set for the time lengths of allrecording marks. More preferably Pb_(i)/Pw≦0.5 and still more preferablyPb_(i)/Pw_(i)≦0.2. The bias power Pb can be set equal to the power Pr ofthe light radiated during retrieving. This simplifies the setting of thedivided pulse circuit required for the pulse division.

For the time length of one particular recording mark, two or moredifferent values of Pb_(i) and/or Pw_(i) may be used according to i.Particularly, setting the recording powers Pw₁ and Pw_(m) used in thestart recording pulse section α₁T and the last recording pulse sectionα_(m)T to values different from the recording power Pw_(i) used in theintermediate recording pulse sections α_(i)T (2≦i≦m−1) can control theshape of the front and rear ends of the mark accurately. It is preferredthat the recording powers Pw_(i) in the intermediate recording pulsesections α_(i)T (2≦i≦m−1) be set equal as practically as possible asthis simplifies the setting of the divided pulse circuit. Similarly, itis preferred that the bias powers Pb_(i) in the off pulse sectionsβ_(i)T (1≦i≦m−1) be all set to the same value as practically as possibleunless there is any justifiable reason. At least two recording markswith different n's may have different values of Pw_(i) and/or Pb_(i) forthe same i.

In this invention, although there are no limiting specifications as towhat power of light shall be radiated onto the spaces where no recordingmarks are formed, the light to be radiated should preferably have apower Pe, which is Pb_(i)≦Pe<Pw_(i). In the rewritable recording medium,the power Pe is an erase power used to erase the recorded marks. In thiscase, it is preferred that during a (n−(η₁+η₂))T section, light with apower equal to or higher than the bias power Pb_(i) and equal to orlower than the erase power Pe be radiated. Setting the light power equalto the bias power Pb_(i) or the erase power Pe facilitates the settingof the divided pulse circuit. When light with the bias power Pb isradiated during an η₁T section, the light with the bias power Pb isradiated prior to the start recording pulse section α₁T, thus minimizingthe influences of heat from the preceding recording mark.

The recording power Pw and bias power Pb or erase power Pe havedifferent physical functions depending on the type of the opticalrecording medium used.

In the case of the magnetooptical medium, for example, Pw or Pe is apower necessary to raise the temperature of the recording layer at leastabove the vicinity of the Curie temperature to make the occurrence ofthe magnetization inversion easy. In the so-called optical modulationoverwritable magnetooptical medium, Pw is greater than Pe and is a powerto raise the temperatures of a plurality of magnetic layers withdifferent Curie points above one of the Curie point temperatures.

In the case of the phase change medium, when performing the recordingthrough crystallization, Pw is a power to raise the recording layer to atemperature higher than the crystallization temperature. Or whenperforming the recording through transformation into amorphous state, Pwis a power to raise the recording layer at least to a temperature higherthan its melting point. When performing overwriting throughamorphisation recording and crystallization erasing, Pe is a power toraise the recording layer at least above the crystallizationtemperature.

In the write-once medium that performs recording through pitting ordeformation of a metallic or organic recording layer, Pw is a powernecessary to raise the recording layer to a temperature that inducessoftening, melting, evaporation, decomposition or chemical reaction.

Although the values of the recording power Pw and bias power Pb differfrom one kind of recording medium used to another, in the rewritablephase change medium for example the recording power Pw is normally about1-100 mW and the bias power Pb about 0.01-10 mW.

Whichever medium is used, the recording power Pw is a laser beam powernecessary to raise the recording layer to a temperature that inducessome optical changes in the recording layer, or to hold thattemperature. The bias power Pb on the other hand is a power at leastlower than the recording power Pw. Normally, the bias power Pb is lowerthan the recording power Pw and the erase power Pe and does not induceany physical changes in the recording layer.

The heat accumulation problem described above is common to a wide rangeof optical disks that perform the mark length modulation recording, suchas phase change type, magnetooptical type and write-once type opticalrecording media.

In the overwritable phase change medium among others, because the markrecording and mark erasing are performed at the same time by preciselycontrolling two temperature parameters, the heating speed and coolingspeed of the recording layer, the function of cooling the recordinglayer by the off pulses bears more importance than in other write-oncemedium and magnetooptical medium. Hence, this invention is particularlyeffective for the phase change type recording medium.

In the recording method using the pulse division of this invention, thesame pulse division number m may be used on at least two recording markswhich have different n's of time lengths nT of the pulse recordingmarks. Normally, the same m values are used for the nT marks havingadjoining time lengths, such as 3T mark and 4T mark. With m values setequal, at least one of α_(i) (1≦i≦m), β_(i) (1≦i≦m), η₁, η₂, Pw_(i)(1≦i≦m) and Pb_(i) (1≦i≦m) is made to differ from others. This makes itpossible to differentiate the time lengths of the marks from one anotherthat have the same division numbers.

The division numbers m may be arranged irrelevant to the magnitudes ofthe n values but it is preferred that the division numbers m be set tomonotonously increase as the mark becomes longer, i.e., the value of mincreases (including the case of staying the same).

Examples of pulse division scheme according to this invention are shownbelow.

Example 1 of Division Scheme

For example, in the EFM modulation that forms 3T to 11T marks, m=1 forn=3 and m is increased for n≧4 (4, 5, 6, 7, 8, 9, 10, 11). That is, thedivision number m is increased tom=1, 2, 2, 3, 4, 5, 6, 7, 8as the n value increases ton=3, 4, 5, 6, 7, 8, 9, 10, 11.

The value of n/m is minimum at 1.38 when n=11 and maximum at 3 when n=3.

Example 2 of Division Scheme

In the same EFM modulation, the division number m is increased tom=1, 2, 2, 3, 4, 5, 6, 6, 6as the n value increases ton=3, 4, 5, 6, 7, 8, 9, 10, 11.

The value of n/m is minimum at 1.5 when n=9 and maximum at 3 when n=3.

Example 3 of Division Scheme

In the same EFM modulation, the division number m is increased tom=1, 2, 2, 3, 3, 4, 5, 5, 5as the n value increases ton=3, 4, 5, 6, 7, 8, 9, 10, 11.

The value of n/m is minimum at 1.8 when n=9 and maximum at 3 when n=3.

When the same pulse division number m is used on at least two recordingmarks with different n values, a pulse period τ_(i)+α_(i)+β_(i) and aduty ratio (α_(i)/(α_(i)+β_(i)) may be changed. Examples of thisprocedure are shown below.

Example 4 of Division Scheme

The simplest division scheme is to make an equal division such that thepulse period τ_(i)=nT/m when m≧2.

However, simply dividing nT into equal parts may result in τ_(i)assuming a value totally irrelevant to the timing and length of thereference clock period T.

Example 5 of Division Scheme

The pulse period τ_(i) is preferably synchronized to the reference clockperiod T or to the reference clock period T divided by an integer(preferably ½T, ¼T, ⅕T, 1/10T) as this allows the rising/falling edge ofthe pulse to be controlled with one base clock taken as a reference. Atthis time, Σ_(i)(τ_(i))=Σ_(i)(α_(i)+β_(i)) does not necessarily agreewith n and an excess time is produced, so that the pulse length must becorrected. It is preferred that the sum of the pulse irradiation timesbe set smaller than n because setting the sum greater than n makes themark length too long.

Hence, sections η₁T, η₂T are provided such thatΣ_(i)(α_(i)+β_(i))+(η₁+η₂)=n (η₁ and η₂ are each real numbers such that0≦η₁ and 0≦η₂), and these sections are changed in each of two recordingmarks that have the same division numbers m but different lengths.During the sections η₁T, η₂T light with the bias power Pb may beradiated. At this time, it is preferred that 0≦(η₁+η₂)≦1.

The above η₁ and η₂ can also be used to correct the effect of heattransferred from other preceding and/or subsequent marks. In this case,the time lengths of η₁T and η₂T are made variable according to the marklengths and/or space lengths of the preceding and/or subsequent marks.

It is possible to use only the first η₁T or last η₂T of the dividedpulses and set the other to 0, or to use both of them in the range of0≦(η₁+η₂)≦1. It is also possible to radiate light having other than thebias power Pb during the sections η₁T, η₂T to align the mark lengths orto more precisely control the influence of heat transferred from thepreceding and/or subsequent marks.

Example 6 of Division Scheme

The divided pulse period τ_(i) and the duty ratio (α_(i)/(α_(i)+β_(i)))are made variable according to i. With this method, jitters(fluctuations) in the front and rear ends of the mark, which areimportant in the mark length recording, can be improved.

More specifically, the first recording pulse period τ₁ and/or the lastrecording pulse period τ_(m) are made to differ from a recording pulseperiod τ_(i) (2≦i≦m−1) of intermediate pulses.

At this time it is possible to slightly adjust τ₁, α₁, β₁, τ_(m), α_(m)and β_(m) of the first and/or last pulse according to the precedingand/or subsequent mark length or space length.

It is preferred that the first recording pulse section α₁T be set largerthan any of the subsequent recording pulse sections α₂T, . . . , α_(m)T.It is also preferred that the recording power Pw₁ be set higher than therecording power Pw_(i) in the succeeding recording pulse sections α₂T, .. . , α_(m)T. These methods are effective in improving an asymmetryvalue of the retrieve signal described later.

The heat accumulation effect is small in short marks such as those withtime lengths of 3T and 4T so that the mark tends to be formed slightlyshorter than required. In such a case, the mark time length may bestrictly controlled by elongating the recording pulse section α₁T tosome extent or setting the recording power Pw₁ in the recording pulsesection α₁T slightly higher than required.

The method of changing the first pulse or last pulse is particularlyeffective when overwriting an amorphous mark in the crystal area of thephase change medium.

Changing the first recording pulse section α₁T can control the width ofan area of the recording layer in the phase change medium that firstmelts.

The last off pulse section β_(m)T is important in preventing therecording layer of the phase change medium from getting recrystallizedand is also an important pulse that determines the area in which therecording layer is made amorphous.

When an amorphous mark is formed, an area in the rear end part of themark that has melted crystallizes again, making the actually formedamorphous mark smaller than the melted area. Elongating the off pulsesection, i.e., extending the cooling time length, can preventrecrystallization and elongates the amorphous portion. Hence, bychanging the length of the last off pulse section β_(m)T it is possibleto change the time during which the rear end portion of the mark is keptin the crystallization time and thereby change the mark length insignificant degrees.

Conversely, by changing the intermediate parameters τ_(i), α_(i), β_(i)(2≦i≦m−1) without changing τ₁, α₁, β₁, τ_(m), α_(m) and β_(m), thedegree of modulation can be controlled without affecting the mark edges.

Now, the method of generating divided recording pulses that realizes theabove-described division scheme will be explained below.

The above pulse division can basically be realized by making thedivision scheme for each mark time length nT programmable andincorporating it into a ROM chip. However, adding a very wide range offlexibility to the same pulse generating circuit will render the circuitcomplex. So, the following two pulse generating methods may preferablybe used. They can provide pulses capable of dealing with almost allmedia with ease.

Divided Recording Pulse Generating Method 1

For the mark length modulated data 100, which is EFM-modulated as shownin FIG. 1(a), a division scheme 101 shown in FIG. 1(b) is applied. Thatis, the division is made as follows:m=1, 1, 2, 3, 3, 4, 5, 5, 5 for n=3, 4, 5, 6, 7, 8, 9, 10, 11.At this time, the circuits Gate1, Gate2, Gate3, Gate4 that generateclocks at timings shown in FIG. 1(c) are combined to realize thedivision scheme of FIG. 1(b).

In FIG. 1(c), the Gate1 denoted 102 generates the first recording pulseα₁T with a delay time of T_(d1). The Gate2 denoted 103 generates a groupof second and subsequent intermediate recording pulses α_(i)T with adelay time of T_(d2). The Gate3 denoted 104 generates pulses with a biaspower Pb and pulses with power Pe. That is, when recording pulses arenot generated by the Gate1, Gate2 and Gate4, off pulses β_(i)T with abias power Pb aretrieved when the level is low and pulses with a powerPe aretrieved when the level is high. The Gate3 and T_(d1) determine(n−(η₁+η₂))T. The Gate4 denoted 105 generates a last recording pulseα_(m)T with a delay time of T_(d3) after the intermediate recordingpulse group α_(i)T has been generated. In the sections in which theGate3 is at low level, when the recording pulses are at high level, theyhave priority over the off pulses.

β₁T can be controlled independently by the delay time T_(d2) and α₁T,and β_(m)T can be controlled independently by Gate3 and α_(m)T.

In the section where the α₁T pulse is generated by the Gate1, arecording power Pw₁ is used; in the sections where the intermediatepulse group α_(i)T is generated by the Gate2, a recording power Pw₂ isused; and in the section where the α_(m)T pulse is generated by theGate4, a recording power Pw₃ is used. This arrangement allows therecording power to be controlled independently in each of the firstpulse section, the intermediate pulse section group and the last pulsesection.

To independently control the recording pulse width and the recordingpower in the first and last sections, the period of the intermediatepulses is defined by γ_(i)=α_(i)+β_(i−1) (2≦i≦m−1) with T_(d2) as astart point, and γ_(i) is set almost constant at γ_(i)=1 to 3. In thiscase, β_(i) is automatically determined. In FIG. 1, γ_(i)=1.5. It isnoted, however, that T_(d2) is defined so as to make a correction of(T_(d2)−(T_(d1)+α₁T)) for β₁ and thus β₁ can be handled as anindependent parameter.

In either case, it is assumed that the Gate timing is synchronized withthe reference clock period T or with a base clock, which is thereference clock period divided by an integer, and that α_(i) and β_(i)are defined by the duty ratio with respect to the base clock.

If n is smaller than a predetermined value n_(c), then m=1 and theintermediate pulse group are not generated by the Gate2. If n is equalto or larger than n_(c), a predetermined number of pulses aretrievedaccording to the above (division scheme example 3). In FIG. 1, n_(c) isset to 5 and when n is equal to or smaller than 4, then m=1; and when nis 5 or more, the intermediate pulses are generated. Here, it is assumedthat the intermediate pulses are generated, according to n, in numbersequal to the division number stored in the ROM memory.

The last pulse α_(m)T generated by the Gate4 is generated only whenn≧n_(c)+1. This is indicated by, a 9T mark in FIG. 1.

When n=n_(c), the pulse is divided into two pulses, the first pulse andone intermediate pulse. In FIG. 1 this is represented by a 5T mark.

When a plurality of marks with different time lengths are each dividedinto the same number of divisions, if a 3T mark and a 4T mark in FIG. 1for example are both recorded by a pair of recording pulse and an offpulse, at least α₁, β₁, η₁ and η₂ and, if further required, Pw₁ and Pw₃need to be differentiated between the 3T mark and the 4T mark.

Divided Recording Pulse Generating Method 2

The following description concerns a divided recording pulse generatingmethod based on a clock signal with a period of 2T which is obtained bydividing the reference clock period T. This method has more limitationsthan the divided recording pulse generating method 1 but has anadvantage of allowing for the design of logic circuits based on moreregular rules.

The pulse generating method 2 is characterized in that the proceduredepends on whether the value that n of an nT mark can take is odd oreven.

That is, for the recording of a mark in which n is even, i.e., the marklength is nT=2LT (L is an integer equal to 2 or more), the mark isdivided into the number of sections m=L and the α₁ and β₁ in therecording pulse sections α₁T and the off pulse sections β₁T are definedas follows.α₁+β₁=2+δ₁α_(i)+β_(i)=2(2≦i≦m−1)α_(m)+β_(m)=2+δ₂(where δ₁ and δ₂ are real numbers that satisfy −0.5≦δ₁≦0.5 and −1≦δ₂≦1;and when L=2, it is assumed that only α₁, β₁, α_(m) and β_(m) exist).

For the recording of a mark in which n is odd, i.e., the mark length isnT=(2L+1)T, on the other hand, the mark is divided into the number ofsections m=L and the α_(i)′ and β_(i)′ in the recording pulse sectionsα_(i)′T and the off pulse sections β_(i)′T are defined as follows.α₁′+β₁′=2.5+δ₁′α_(i)′+β_(i)′=2(2≦i≦m−1)α_(m)′+β_(m)′=2.5+δ₂′(where δ₁′ and δ₂′ are real numbers that satisfy −0.5≦δ₁′≦0.5 and−1≦δ₂′≦1; and when L=2, it is assumed that only α₁′, β₁′, α_(m)′ andβ_(m)′ exist).

Further, in the pulse generating method 2, the following equation issatisfied.α₁+β₁+α_(m)+β_(m)+Δ=α₁′+β₁′+α_(m)′+β_(m)′(where Δ=0.8 to 1.2).

In the above pulse generating method 2, α₁, β₁, α₁′, β₁′, δ₁, δ₂, δ₁′,and δ₂′ may change according to the value of L. In the pulse generatingmethod 2, in the process of forming recording marks with n=2L andn=(2L+1), they are both divided into the same division number L ofrecording pulses. That is, when n is 2, 3, 4, 5, 6, 7, 8, 9, . . . , inthat order, then the division number m is set to 1, 1, 2, 2, 3, 3, 4, 4,. . . in that order. More specifically, in the EFM modulation signal,for n=3, 4, 5, 6, 7, 8, 9, 10, 11, the division number m is sequentiallyset to m=1, 2, 2, 3, 3, 4, 4, 5, 5 in that order. In the EFM+ signal,n=14 is added. In that case, the division number m is set to 7. In the(1, 7)-RLL-NRZI modulation there is a case of n=2, in which case thedivision number m is set to 1.

In the pulse generation method 2, two recording marks with the samedivision numbers m=L and different lengths have only the first pulseperiod (α₁+β₁)T and the last pulse period (α_(m)+β_(m))T differ fromeach other. That is, for (α₁+β₁+α_(m)+β_(m)), (α₁′+β₁′+α_(m)′+β_(m)′) isincreased by Δ(Δ=0.8 to 1.2). The Δ is normally 1 but can be changed ina range of about 0.8 to 1.2, considering the influence of heatinterference from the preceding and subsequent recording marks.

δ₁ and δ₂, and δ₁′ and δ₂′ are adjusted to ensure that each mark lengthwill be precisely nT and to reduce jitters at the ends of the mark. Theyare normally −0.5≦δ₁≦0.5, −0.5≦δ₁′≦0.5, −1≦δ₂≦1 and −1≦δ₂′≦1. Thecorrection amounts at the front end and rear end are preferably setequal, i.e., |δ₂/δ₁| and |δ₂′/δ₁′| are each preferably in the range of0.8 to 1.2.

The two recording marks with the same division numbers are preferablyformed in such a manner that their mark length difference 1T is about0.5T at the front end side and about 0.5T at the rear end side. That is,α₁+β₁+Δ₁=α₁′+β₁′(where Δ₁=0.4 to 0.6)In this case, the rear end side is normallyα_(m)+β_(m)+Δ₂=α_(m)′+β_(m)′(where Δ₂=0.4 to 0.6 and Δ₁+Δ₂=Δ)

Setting δ₁=about 0 and δ₁′=about 0 is particularly preferred as thisallows the use of a circuit that can generate divided pulses insynchronism with the front end of the mark. The position of the frontend of the mark is determined almost by the rising edge of the recordingpower laser beam at α₁T and its jitter is determined by the duty ratioof α₁ and β₁ and by the duty ratio of α₁′ and β₁′. Hence, in thismethod, setting δ₁=0 and δ₁′=0.5 can control the mark front end positionand the jitter satisfactorily.

The mark rear end position depends on δ₂ (and δ₂′), i.e., the value ofthe divided pulse period (α_(m)+β_(m))T (and (α_(m)′+β_(m)′)T) at therear end of the mark and also on the value of the duty ratio of α_(m)and β_(m) (and the duty ratio of α_(m)′ and β_(m)′). Further, the markrear end position also depends on the position of the falling edge ofthe recording pulse α_(m)T (and β_(m)′T) at the rear end and on thecooling process of the recording layer before and after that fallingedge position. In the phase change medium where amorphous marks areformed, in particular, the mark rear end position depends on the valueof the off pulse section β_(m)T (and β_(m)′T) at the rear end that has agreat effect on the cooling speed of the recording layer. Hence, thedivided pulse period (α_(m)+β_(m))T at the rear end does not need to be0.5T or 1T, and fine adjustment can be made with a resolution power ofabout 0.1T, preferably 0.05T, or more preferably 0.025T.

In the pulse generating method 2, the duty ratio between α_(i) andβ_(i), α_(i)/(α_(i)+β_(i)), can be optimized for each mark length, butfor simplification of the pulse generating circuit, it is preferred thatthe duty ratios in the intermediate pulses situated between the firstpulse and the last pulse be set to a fixed value. That is, when L≧3 inwhich case intermediate pulses can exist, it is preferred that, for alli ranging from 2 to (m−1) in two recording marks with the same divisionnumbers m=L, α_(i) and α_(i)′ be set to α_(i)=αc (fixed value) andα_(i)′=αc′ (fixed value). Further, when L is 3 or more, αc and αc′ arepreferably set to a fixed value, particularly αc=αc′, not dependent onthe value of L because this further simplifies the circuit.

For the simplified pulse generating circuit in the pulse generatingmethod 2, it is preferred that in the recording mark with n being even,α₁ and β₁ assume fixed values for all L equal to 3 or more. For all Lequal to 2 or more, it is preferred that α₁+β₁ be set to 2 as thiscauses the period (α_(i)+β_(i))T to become 2T for all i ranging from 1to (m−1).

Similarly, for the simplified pulse generating circuit in the pulsegenerating method 2, it is preferred that in the recording mark with nbeing odd, α₁′ and β₁′ assume fixed values for all L equal to 3 or more.For all L equal to 2 or more, it is preferred that α₁+β₁ be set to 2.5as this makes it easy to synchronize with the subsequent divided pulseperiod 2T.

Further, for the simplification of the pulse generating circuit in thepulse generating method 2, α_(m), β_(m), α_(m)′ and β_(m)′ eachpreferably assume the same values for all L equal to 3 or more,specifically 2 or more. Here, if Δ₂=(α_(m)′+β_(m)′)−(α_(m)+β_(m))=0.5,the circuit can be further simplified.

When n is 2 or 3, the division number m is 1. In that case, the α₁−β₁duty ratio and δ₁ (or the α₁′−β₁′ duty ratio and δ₁′) can be adjusted toachieve a desired mark length and jitter. Here, it is desired thatδ₁′−δ₁=1.

In the pulse generating method 2, as described above, it is particularlydesired that δ₁=δ₁′=0. In that case, the pulse generating circuit shouldpreferably be controlled to ensure that α_(i) (1≦i≦m) is generated insynchronism with a frequency-divided first reference clock 3 with aperiod 2T which is produced by frequency-dividing a first referenceclock 1 with a period T; that α_(i)′ (2≦i≦m) is generated in synchronismwith a frequency-divided second reference clock 4 with a period 2T whichis obtained by frequency-dividing a second reference clock 2 that hasthe same period T as that of the first reference clock 1 and is shifted0.5T from the first reference clock 1; and that α₁′ rises 2.5T beforeα₂′ rises. The use of a plurality of reference clocks can simplify thepulse generating circuit.

There is a case in which the rising edges of α₁ and α₁′ need to bedelayed or advanced with respect to the rising or falling edge of asquare wave modulated according to the mark length to be recorded. Insuch a case it is preferred that the same delay times T_(d1) be added inorder to make the lengths of spaces constant. T_(d1) is a real numberbetween −2 and 2. When the value of T_(d1) is negative, it indicates aadvance time.

FIG. 12 shows an example relation between recording pulses when thepulse division scheme in the recording method of this invention isimplemented by using a plurality of reference clocks described above. InFIG. 12, the delay times T_(d1) of α₁T and α₁′T with respect to thefront end of the nT mark are 0; the recording power in the recordingpulse section α_(i)T (1≦i≦m) is Pw which is constant; the bias power inthe off pulse section β_(i)T (1≦i≦m) is Pb which is constant; and thepower of light radiated in the spaces and in other than α_(i)T (1≦i≦m)and β_(i)T (1≦i≦m) is an erase power Pe which is constant. HerePb≦Pe≦Pw.

In FIG. 12, reference number 200 denotes a reference clock with a periodT.

FIG. 12(a) shows a pulse waveform corresponding to a recording mark witha length of nT, with reference number 201 representing the length of a2LT recording mark and 202 representing the length of a (2L+1)Trecording mark. FIG. 12(a) illustrates a case of L=5.

FIG. 12(b) shows a divided recording pulse waveform when n=2L (=10) andFIG. 12(c) shows a divided recording pulse waveform when n=2L+1 (=11).

In FIG. 12(b), a frequency-divided first reference clock 205 with aperiod 2T is obtained by frequency-dividing a first reference clock 203which has a zero phase delay from the reference clock 200 with a periodT. Because α₁+β₁=2, the rising edge of each recording pulse sectionα_(i)T (1≦i≦m) is synchronized with the frequency-divided firstreference clock 205 with a period 2T. In synchronism with thefrequency-divided first reference clock 205, the duty ratio ofα_(i)−β_(i) is adjusted to produce a recording pulse waveform 207.

In FIG. 12(c), a frequency-divided second reference clock 206 with aperiod 2T is obtained by frequency-dividing a second reference clock 204with a period T which has a phase shift of 0.5T from the reference clock200. The leading edge of each recording pulse section α_(i)T (2≦i≦m) issynchronized with the frequency-divided second reference clock 206 witha period 2T. Because α₁+β₁=2.5, only α₁T rises 0.5T before the clock. Insynchronism with the frequency-divided second reference clock 206, theduty ratio of α_(i)−β_(i) is adjusted to produce a recording pulsewaveform 208.

In FIG. 12, the mark lengths 2LT and (2L+1)T are depicted so that theirrear ends are aligned at T2 and T4. Hence, there are only two possiblerelations (b) and (c) between the reference clocks 205 and 206, bothwith the period of 2T. In reality, however, when the 2T-period referenceclocks are used, the front end positions of these mark lengths can be 1Tout of phase with each other. Further, considering the cases of n beingeven and n being odd, there are four possible relations as shown inFIGS. 13(a), (b), (c) and (d). It is therefore desirable to adopt thefollowing gate generating method to deal with this situation.

FIG. 13 is a timing chart explaining the above gate generating method.The gate generating method of FIG. 13 involves the following steps: (1)it generates a reference time T_(sync) corresponding to the clock markformed at a predetermined position on the recording track; (2) itgenerates four reference clocks, a 2T-period reference clock 1a laggingthe reference time T_(sync) as a start point by the delay time T_(d1), a2T-period reference clock 2a leading the reference clock 1a by 0.5T, a2T-period reference clock 1b leading the reference clock 1a by 1T, and a2T-period reference clock 2b leading the reference clock 1a by 1.5T; (3)when recording a mark of nT=2LT, it generates gate groups G1a, G1b insynchronism with either the reference clock 1a or 1b at timingscorresponding to the α₁T, α_(i)T (2≦i≦m−1) and α_(m)T sections; and (4)when recording a mark of nT=(2L+1), it generates gate groups G2a, G2b insynchronism with either the reference clock 2a or 2b at timingscorresponding to α₁′T, α_(i)′T′ (2≦i≦m−1) and α_(m)′T.

In FIG. 13 the reference number 300 represents the reference clock witha period T (reference clock for data). To record data at a particularaddress on the recording medium, the recording system normally makes aphase comparison between the synchronization signal T_(sync) generatedat clock marks arranged on the medium for each minimum unit of address(e.g., synchronization signal such as VFO formed as a pit train on themedium and arranged for each sector, and a synchronization patternarranged for each ATIP frame (every 1/75 second) formed as a groovemeander on the medium) and the T-period reference clock generated inadvance in order to establish synchronization.

FIG. 13 shows an example case in which the front end of the mark appearsan even number of periods T after the T_(sync). An even-numbered lengthmark 301 with n being even is shown in FIG. 13(a) and an odd-numberedlength mark 304 with n being odd is shown in FIG. 13(d). As an examplein which the front end of the mark appears an odd number of periods Tafter the T_(sync), an even-numbered length mark 302 with n being even(FIG. 13(b)) and an odd-numbered length mark 303 with n being odd (FIG.13(c)) are shown.

In each of FIGS. 13(a) to 13(d), when reference clocks are generated byshifting them 0.5T from one another in a manner similar to that of FIG.12, four different clock trains are produced, as indicated by 305, 306,307 and 308. That is, with the reference clock 305 taken as a reference,the clock 307 is shifted by 0.5T, the clock 306 is shifted by 1T, andthe clock 308 is shifted by 1.5T. These clocks are all formed byfrequency-dividing the T-period reference clock having its origin atT_(sync) and then shifting their phases by 0.5T from one another.

In the case of FIG. 13(a), in synchronism with each of periods P1a, P2a,P3a, P4a and P5a the gate group G1a corresponding to the recording pulsesections α₁T, α₂T, α₃T, α₄T, α₅T.

In the case of FIG. 13(b), in synchronism with each of periods P1b, P2b,P3b, P4b and P5b the gate group G1b corresponding to the recordingpulses α₁T, α₂T, α₃T, α₄T, α₅T.

In the case of FIG. 13(c), in synchronism with each of periods R1a, R2a,R3a, R4a and Rya the gate group G2a corresponding to the recordingpulses α₁′T, α₂′T, α₃′T, α₄′T, α₅′T.

In the case of FIG. 13(d), in synchronism with each of periods R1b, R2b,R3b, R4b and R5b the gate group G2b corresponding to the recordingpulses α₁′T, α₂′T, α₃′T, α₄′T, α₅′T.

Here, the recording pulse generating gate groups G1a, G1b, G2a and G2bare identical to the Gate1, Gate2 and Gate4 combined in FIG. 1. That is,in FIG. 1, the Gate1 for generating the first pulse α₁T, the Gate2 forgenerating the intermediate pulse group α_(i)T (2≦i≦m−1), and the Gate4for generating the last pulse α_(m)T are produced separately and thencombined to generate the gate groups G1a and G1b. In FIG. 1, the firstpulse α₁′T, the intermediate pulse group α_(i)′T (2≦i≦m−1), and the lastpulse α_(m)′T are produced separately and then combined to generate thegate groups G2a and G2b.

Generating the first pulse independently as with the Gate1 of FIG. 1 candeal with the situation where (α₁′+β₁′) is 2.5 when n is odd, bygenerating the gate for α₁′T in synchronism with the front end of nT andgenerating the 2T-period intermediate pulse group α_(i)′T with a delayof 2.5T. This is equivalent to setting the T_(d2) for Gate2 in FIGS. 1to 2.5T (when there is a delay T_(d1,) another delay T_(d1) is made).

The gate groups G1a, G1b, G2a and G2b are selected as follows. First,with T_(sync) taken a reference, the starting point of the T-periodreference clock 300 is determined, and it is checked whether the marklength nT rises an even number of clock periods T or an odd number ofclock periods T after the starting point. More specifically, a 1-bitadder is used which is reset at T_(sync) and adds 1 every period. If theresult is 0, it is decided that the elapsed time is determined to be aneven number of periods; and if the result is 1, the elapsed time isdetermined to be an odd number of periods. That is, if the elapsed timefrom the reference time T_(sync) to the front end of the nT mark is aneven number times the period T, then the gate signal group G1a or G2b isselected depending on whether n is even or odd. If the elapsed time fromthe reference time T_(sync) to the front end of the nT mark is an oddnumber times the period T, then the gate signal group G1b or G2a isselected depending on whether n is even or odd. It is therefore possibleto generate all the recording pulses in a series of nT marks which aregenerated, with T₀ as a starting point, by using combinations of thefour 2T-period reference clocks shifted 0.5T from one another.

To determine the length of the off pulse section and the erase power Pelight irradiating section requires focusing attention on the off pulsesection β_(m)T. That is, it is desired to provide the rear end of themark with not the period 2T but with a margin of about ±1T. In thiscase, the timing of the last off pulse β_(m) or β_(m)′ needs to bedefined exceptionally. To this end, it is preferable to generate a gatesignal corresponding to the Gate3 of FIG. 1. For example, with the frontend of the nT mark taken as a reference, the gate signals are generateddepending on whether n is even or odd, that is, a gate G3 ofΣ(α_(i)+β_(i)) is generated with a delay time of T_(d1) when n is even;and a gate G4 of Σ(α_(i)′+β_(i)′) is generated with a delay time ofT_(d1) when n is odd, to radiate light with different powers accordingto the following conditions. (1) in a duration where both G3 and G4 areoff, light with a power Pe is radiated; (2) in a duration where eitherG3 or G4 is on, light with a power Pb is radiated; (3) in a durationwhere both G3 and G1a are on at the same time, light with a power Pw isradiated for a G1a-on section; (4) in a duration where both G3 and G1bare on at the same time, light with a power Pw is radiated for a G1b -onsection; (5) in a duration where both G4 and G2a are on at the sametime, light with a power Pw is radiated for a G2a-on section; and (6) ina duration where both G4 and G2b are on at the same time, light with apower Pw is radiated for a G2b-on section.

The gate priority relationship described above is determined by matchingthe gate on/off to logical 0 and 1 levels and performing an OR operationon each gate controlling logical signal.

FIG. 12 and FIG. 13 represent a case where, for simplicity, the risingedge of the first recording pulse α₁T, α₁′T is at the front end of thenT mark, i.e., concurrent with the front end of the nT mark beingrecorded. If the mark has a delay, it is preferred in terms of keepingthe space length at a desired value that the rising edges of α₁T andα₁′T be provided with the same delay T_(d1).

Divided Recording Pulse Generating Method 3

The following description concerns another example of the dividedrecording pulse generating method based on a 2T-period clock signalwhich is obtained by dividing the reference clock period T This methodallows for the design of logic circuits based on more regular rules thanthose employed in the divided recording pulse generating method 1.

In more concrete terms, as in the pulse generating method 2, theprocedure depends on whether the value the n of an nT mark can take isodd or even. In the divided recording pulse generating method 2, thecorrection of the mark length difference 1T between an even-numberedlength mark and an odd-numbered length mark, both having the same numberof divisions, is distributed and allocated to the first and lastrecording pulse periods. In the pulse generating method 3, however, thecorrection of the mark length difference 1T is done by adjusting the offpulse length β_(i)T (2≦i≦m−1) in the intermediate divided recordingpulse group.

That is, for the recording of a mark in which n is even, i.e., the marklength is nT=2LT (L is an integer equal to 2 or more), the mark isdivided into the number of sections m=L and the α_(i) and β_(i) in therecording pulse sections α_(i)T and the off pulse sections β_(i)T aredefined as follows.T_(d1)+α₁=2+ε₁β_(i−1)+α_(i)=2(2≦i≦m)

For the recording of a mark in which n is odd, i.e., the mark length isnT=(2L+1)T, on the other hand, the mark is divided into the number ofsections m=L and the α_(i)′ and β_(i)′ in the recording pulse sectionsα_(i)′T and the off pulse sections β_(i)′T are defined as follows.T_(d1)′+α₁′=2+ε₁′β₁′+α₂′=2.5+ε₂′β_(i−1)′+α₁′=2(3≦i≦m−1)β_(m−1)′+α_(m)′=2.5+ε₃′(When L=2, it is assumed that β₁′+α₂′=2.5+ε₂′ or β₁′+α₂′=3+ε₂′)

Then, β₁, α₂, β_(m−1), α_(m), β₁′, α₂′, β_(m−1) and α_(m)′ satisfy thefollowing equation.β₁+α₂+β_(m−1)+α_(m)+Δ₂=β₁′+α₂′+β_(m−1)′+α_(m)′(where Δ₂=0.8 to 1.2).

The values of α_(i), β_(i), α_(i)′, β_(i)′, T_(d1), T_(d1)′, ε₁, ε₁′,ε₂′ and ε₃′ can vary according to L.

T_(d1) and T_(d1)′ are delay or advance times from the starting end ofthe nT mark in the mark length-modulated original signal until the firstrecording pulse α₁T rises. They are real numbers normally between −2 and2. The positive values of T_(d1) and T_(d1)′ signify delays. T_(d1) andT_(d1)′ are preferably set almost constant regardless of the value of L.

α_(i), β_(i), α_(i)′ and β_(i)′ are real numbers normally between 0 and2, preferably between 0.5 and 1.5.

ε₁, ε₁′, ε₂′ and ε₃′ are real numbers normally between −1 and 1,preferably between −0.5 and 0.5. These are used, as required, ascorrection values for realizing precise mark lengths or space lengths inthe divided pulse periods (β_(i−1)+α_(i))T that form the period 2T.

In the pulse generating method 3, two marks corresponding to n=2LT andn=(2L+1)T, where L's are equal, are divided into the same divisionnumber L of recording pulses in the process of recording. That is, forn=2, 3, 4, 5, 6, 7, 8, 9, . . . , the number of recording pulses for thecorresponding n is set to 1, 1, 2, 2, 3, 3, 4, 4, 5, 5 . . . Forexample, in the EFM modulation signal, for n=3, 4, 5, 6, 7, 8, 9, 10,11, the division number m is sequentially set to m=1, 2, 2, 3, 3, 4, 4,5, 5 in that order. In the EFM+ signal, n=14 is added. In that case, thedivision number m is set to 7. In the (1, 7)-RLL-NRZI modulation, thedivision number m is set to 1 also in the case of n=2.

To record two kinds of mark lengths of n=2L and 2L+1 with the samedivision numbers, the period (β₁+α₂)T and the period (β_(m−1)+α_(m))Tare each increased or decreased by 0.5T to adjust their lengths. What isimportant in the mark length recording is the mark end position and thejitter that are determined by the waveform of the front and rear ends ofthe mark. The intermediate portion of the mark does not have a greateffect on the jitter at the ends of the mark as long as the correctamplitude of the intermediate portion is obtained. The above adjustingmethod takes advantage of the fact that as long as the mark does notappear optically divided, if the recording pulse period in theintermediate portion of the mark is extended or reduced by 0.5T, themark length only increases or decreases by the corresponding amount anddoes not greatly affect the jitter at the ends of the mark.

In the pulse generating method 3, 2T is taken as the base recordingpulse period for any mark length. The duty ratio of α_(i)−β_(i) can beoptimized for each mark length or for each i, but it is preferred thatthe following restrictions be provided for the simplification of therecording pulse generating circuit.

As to the front end of the mark, α₁, β₁, α₁′ and β₁′ are each preferablymade constant and independent of L for the L value of 3 or more. Morepreferably, α₁′=0.8α₁ to 1.2α₁ and β₁′=β₁+about 0.5. Still morepreferably, β₁′=β₁+0.5, α₁=α₁′ and β₁=β₁′. The position of the front endof the mark is almost determined by the leading edge of the firstrecording pulse. That is, if the position of the leading edge ofα₁T=α₁′T is set to lag the starting end of the mark length nT by aconstant delay time T_(d1), the actual front end position of the mark isdetermined almost uniquely. As to the jitter at the front end of themark, on the other hand, if β₁T has more than a certain length (inpractice, 0.5T, ) assuming that α₁T is nearly equal to α₁′T, the jittercan be kept within a satisfactory level irrespective of n value bysetting only β₁′ to approximately β₁′=β₁+0.5.

As to the rear end of the mark, α_(m), β_(m), α_(m)′ and β_(m)′ are eachpreferably made constant and independent of L for the L value of 3 ormore. More preferably, β_(m−1)′=β_(m−1)+about 0.5, α_(m)′=0.8α_(m) to1.2α_(m), and β_(m)′=0.8β_(m) to 1.2β_(m). Still more preferably,β_(m−1)′=β_(m−1)+0.5, α_(m)=α_(m)′, and β_(m)=β_(m)′.

When L=2, it is preferred that β₁′=β₁+0.5 to 1.5, α_(m)′=α_(m)+0 to 1,and β_(m)′=0.8β_(m) to 1.2β_(m). However, in either case, it is desiredthat the falling edges of the α_(m)T and α_(m)′T and the rear end of themark length nT be synchronized, with a predetermined time differencetherebetween.

The rear end position of the mark depends not only on the trailing edgeposition of the last recording pulse α_(m)T (or α_(m)′T) but also on thecooling process of the recording layer temperature before and after themark rear end position. In the phase change medium that forms amorphousmarks in particular, the mark rear end position depends on the coolingspeed of the recording layer temperature controlled by the last offpulse section β_(m)T (or β_(m)′T) Hence, if α_(m)T and α_(m)′T areshifted a predetermined time from the rear end of the nT mark andβ_(m)′=β_(m), then the mark rear end position is determined almostuniquely.

As to the jitter at the rear end, on the other hand, if β_(m−1),β_(m−1)′, α_(m) and α_(m)′ are longer than a predetermined length, thejitter produced is small and governed mostly by β_(m)′=β_(m). Optimizingthe β_(m)′=β_(m) can produce nearly the best jitter.

In the pulse generating method 3, too, in the process of high densityrecording in particular, the values of T_(d1), α₁, α₁′, β₁, β₁′, α_(m),α_(m)′, β_(m) and β_(m)′ can be finely adjusted in the range of about±20% to correct the heat interference according to marks or spacesimmediately before or after the mark being recorded. In the aboveexplanation, the expression “about 0.5” or “about 1” means that the fineadjustment of that degree is allowed.

For further simplification of the pulse generating circuit, when L is 3or more, α_(i) and α_(i)′ are made constant and independent of i for thei value of 2≦i≦m−1. That is,α₂=α₃= . . . =α_(m−1)α₂′=α₃′= . . . =α_(m−1)′Here, the expression “L is 3 or more” is the condition to establish thatthe division number is 3 or more and there is one or more intermediatedivided recording pulses excluding first and last divided pulses.

More preferably, when L is 3 or more, the values of α_(i) and α_(i)′ for2≦i≦m−1 are fixed to constant values of αc and αc′ respectively, whichare independent of L. Still more preferably, αc=αc′. In the mark lengthrecording, the formation of the intermediate portion of the mark haslittle effect on the mark end position and the jitter as long as theappropriate signal amplitudes are produced. In most cases therefore itis possible to make a uniform setting of α_(i)=α_(i)′=αc (2≦i≦m−1) asdescribed above.

It is more preferred that α_(m) and α_(m)′ be set to the same values ofα_(i) and α_(i)′ for 2≦i≦m−1.

When L=1, i.e., the mark length nT is 2T or 3T, it is preferred thatm=1. In that case, the period (α₁+β₁)T and the duty ratio of α₁−β₁ (orperiod (α₁′+β₁′)T and α₁′−β₁′ duty ratio) are adjusted to realize adesired mark length and jitter. If β₁ or β₁′ is constant for n≧4, it ispreferred that β_(m) or β_(m)′ also use the same values of β₁ or β₁′ forn≧4.

These divided recording pulses, when 0≦T_(d1)≦2 and 0≦T_(d1)′≦2, can beformed as follows.

First, (1) it is assumed that an original mark length modulation signalis generated in synchronism with the first reference clock with a periodT. With the starting end of the nT mark of the mark length modulationsignal taken as a reference, the first recording pulse α₁T (or α₁′T) isgenerated with a delay time of T_(d1) (or T_(d1)′). Next, (2) the lastrecording pulse α_(m)T (α_(m)′T) is generated so that its falling edgealigns, after a time difference of ε₃ (or ε₃′), with the rear end of thenT mark. Then, (3) as to α_(i)T and β_(i)T (2≦i≦m−1)—the intermediatedivided recording pulses that are produced when L is 3 or more—α₂T falls4T after the starting end of the nT mark and thereafter α_(i)+β_(i−1)are generated with a period of 2T. (4) When n is odd (n=2L+1), α₂′Tfalls 4.5T after the starting end of the nT mark and thereafterα_(i)′+β_(i−1)′ are generated.

In the above example, also when ε₁, ε₁′, ε₂′ and ε₃′ are not 0, thefalling edge of at least α₂T or α₂′T in the intermediate dividedrecording pulse group is produced precisely the delay time of 4T or 4.5Tafter the starting end of the nT mark. Therefore, at least theintermediate divided recording pulse group can be generated insynchronism with the 2T-period reference clock, which was generated byfrequency-dividing the T-period reference data clock in advance.

FIG. 24 shows the relation between the recording pulses when therecording pulse dividing method of this invention is implemented bycombining a plurality of 2T-period reference clocks.

In FIG. 24, for simplicity, the recording power Pw of light radiatedduring the recording pulse sections, the bias power Pb of light radiatedduring the off pulse sections, and the erase power Pe of light radiatedduring other than these sections are each shown to be constant for anyi. Although these powers are shown to have the relationship of Pb<Pe<Pw,these powers may be set to different values depending on the values of nand i. Particularly, the recording power Pw₁ in α₁T and α₁′T and therecording power Pw_(m) in α_(m)T and α_(m)′T may be set different fromthe recording power Pw_(i) other sections α_(i)T (i=2 to m−1).

Further, in FIG. 24, for simplification, it is assumed thatε₁=ε₁′=ε₂′=ε₃′=0, and the first recording pulses α₁T and α₁′T are shownto fall 2T, after the front end of the nT mark being recorded and thefalling edges of α_(m)T and α_(m)′T are shown to coincide with the rearend of the nT mark.

In FIG. 24, reference number 220 represents a T-period reference clock.

FIG. 24(a) shows square waves associated with the nT mark of theoriginal mark length modulation signal, with 221 representing a mark 2LTin length and 222 representing a mark (2L+1)T in length. Here, althoughtwo kinds of marks corresponding to L=5 are shown, it is possible tohandle other cases of the L value by adding or subtracting the period of2T for the intermediate i of 2≦i≦m−1 each time L increments ordecrements by 1.

FIG. 24(b) represents a waveform of divided recording pulses whenn=2L=10 and FIG. 24(c) represents a waveform of divided recording pulseswhen n=2L+1=11.

In FIG. 24(b), the 2T-period reference clock 225 is obtained byfrequency-dividing a T-period reference clock 223 which has no phasedelay with respect to the T-period reference clock 220. WhenT_(d1)+α₁=2, the falling edge of each recording pulse α_(i)T (1≦i≦m) issynchronized with the 2T-period reference clock 225. In synchronism withthe 2T-period reference clock 225, the duty ratio of α_(i)−β_(i) isadjusted to produce a recording pulse waveform 227.

In FIG. 24(c), a 2T-period reference clock 226 is obtained byfrequency-dividing a T-period reference clock 224 which is 0.5T out ofphase with the T-period reference clock 220. The falling edge of eachrecording pulse α_(i)′T (2≦i≦m) is synchronized with the 2T-periodreference clock 226. In synchronism with the reference clock 226, theduty ratio of β_(i−1)−α_(i) is adjusted to produce a recording pulsewaveform 228.

In this way, by using the T-period first reference clock 1 (223) and theT-period second reference clock 2 (224) 0.5T out of phase with theT-period first reference clock, α_(i) (1≦i≦m) is generated insynchronism with the 2T-period reference clock 3 (225) which is obtainedby frequency-dividing the reference clock 1 and α_(i)′ (2≦i≦m−1) isgenerated in synchronism with the 2T-period reference clock 4 (226)which is obtained by frequency-dividing the reference clock 2, therebyproducing the divided recording pulses corresponding to 2L and 2L+1easily.

In FIG. 24, the mark lengths 2LT and (2L+1)T are depicted to have theirrear ends align with each other at T2 and T4. So, there are only twopossible relations (b) and (c) between the 2T-period reference clocks225 and 226. In reality, however, when the 2T-period reference clocksare used, the front end positions of these mark lengths can be 1T out ofphase with each other although they are in phase with the 2T period.Hence, the divided recording pulse generating method 3 needs also toconsider, as in the divided recording pulse generating method 2, thefact that there are four possible relations considering the cases of nbeing even and n being odd as shown in FIGS. 13(a), (b), (c) and (d).

Then, by using the 2T-period clock train 4 of FIG. 13, in the case of(1a), a gate group G1a corresponding to the recording pulse sectionsα₁T, α₂T, α₃T, α₄T, α₅T is generated in synchronism with each of theperiods P1a, P2a, P3a, P4a, P5a; in the case of (1b), a gate group G1bcorresponding to the recording pulses α₁T, α₂T, α₃T, α₄T, α₅T isgenerated in synchronism with each of the periods P1b, P2b, P3b, P4b,P5b; in the case of (2a), a gate group G2a corresponding to therecording pulses α₁′T, α₂′T, α₃′T, α₄′T, α₅′T is generated insynchronism with each of the periods R1a, R2a, R3a, R4a, R5a; and in thecase of (2b), a gate group G2b corresponding to the recording pulsesα₁′T, α₂′T, α₃′T, α₄′T, α₅′T is generated in synchronism with each ofthe periods Q1b, Q2b, Q3b, Q4b, Q5b.

These recording pulse generating gate groups G1a, G1b, G2a, G2b areidentical to the combinations of Gate 1, 2, and 4 in FIG. 1, as in thecase of the divided recording pulse generating method 2.

That is, in generating G1a and G1b, as shown in FIG. 1, the Gate1 forgenerating the first pulse α₁T, the Gate2 for generating theintermediate pulse group α_(i)T (2≦i≦m−1), and the Gate4 for generatingthe last pulse α_(m)T are separately generated and then combined. Or ingenerating G2a and G2b, as shown in FIG. 1, the first pulse α₁′T, theintermediate pulse group α_(i)′T (2≦i≦m−1), and the last pulse α_(m)′Tare separately produced and then combined. When ε₁, ε₁′, ε₂′ and ε₃′ arenot 0, the first recording pulses α₁T, α₁′T may be given a predeterminedtime difference of period P1a, Q1a, P1b or Q1b, and the last recordingpulses α_(m)T, α_(m)′T are given a predetermined time difference ofeither period P5a, P5b, Q5a or Q5b.

On the other hand, to determine the off pulse sections and the Pe powerirradiation sections, one must consider the fact that the last off pulsesection β_(m)T of the mark is irregular. That is, the period of the rearend of the mark is not necessarily 2T and must be given a margin ofabout 2T±1T. This can be dealt with by defining the last off pulse β_(m)or β_(m)′ exceptionally. For that purpose, the gate signal correspondingto the Gate3 of FIG. 1 is generated.

That is, when n is even, a gate G3 of Σ(α_(i)+β_(i))T is generated witha delay time T_(d1) from the front end of the nt mark; and when n isodd, a gate G4 of Σ(α_(i)′+β_(i)′)T is generated with a delay timeT_(d1)′ from the front end of the nT mark. Then, when either G3 or G4 isoff, the light with the erase power Pe is radiated; when either G3 or G4is on, the light with the bias power Pb is radiated; when both G3 andG1a are on simultaneously, the light with the recording power Pw isradiated in response to the G1a-on section; when both G3 and G1b are onsimultaneously, the light with the recording power Pw is radiated inresponse to the G1b-on section; when both G4 and G2a are onsimultaneously the light with the recording power Pw is radiated inresponse to the G2a-on section; and when both G4 and G2b are onsimultaneously, the light with the recording power Pw is radiated inresponse to the G2b-on section. The gate priority relationship describedabove is determined by matching the gate on/off to logical 0 and 1levels and performing an OR operation on each gate controlling logicalsignal.

In summary, all the gates for generating the recording pulse sectionsα_(i)T can be produced by the following procedure. (1) A reference timeT_(sync) corresponding to the clock mark formed at a predeterminedposition on the recording track is generated; (2) four reference clocksare generated: a 2T-period reference clock 1a produced at the referencetime T_(sync) as a starting point, a 2T-period reference clock 2aproduced 0.5T in advance of the reference clock 1a, 2T-period referenceclock 1b produced 1T, in advance of the reference clock 1a, and a2T-period reference clock 2b produced 1.5T in advance of the referenceclock 1a; (3) in recording a mark of nT=2LT, the gate groups G1a and G1bwhich have timings corresponding to the α₁T, α_(i)T (2≦i≦m−1) and α_(m)Tsections are generated in synchronism with either the reference clock 1aor 1b; (4) in recording a mark of nT=(2L+1)T, the gate groups G2a andG2b which have timings corresponding to the α₁′T, α_(i)′T (2≦i≦m−1) andα_(m)′T are generated in synchronism with either the reference clock 2aor 2b.

The gate groups G1a, G1b, G2a, G2b can be selected as follows. First, itis checked whether the mark length nT rises an even number of clockperiods T or an odd number of clock periods T after the reference timeT_(sync) as a start point. More specifically, a 1-bit adder is usedwhich is reset at T_(sync) and adds 1 every period. If the result is 0,it is decided that the elapsed time is determined to be an even numberof periods; and if the result is 1, the elapsed time is determined to bean odd number of periods. That is, if the elapsed time from thereference time T_(sync) to the front end of the nT mark is an evennumber times the period T, then the gate signal group G1a or G2b isselected depending on whether n is even or odd. If the elapsed time fromthe reference time T_(sync) to the front end of the nT mark is an oddnumber times the period T, then the gate signal group G1b or G2a isselected depending on whether n is even or odd. It is therefore possibleto generate all the recording pulses in a series of nT marks which aregenerated, with T₀ as a starting point, by using combinations of thefour 2T-period reference clocks shifted 0.5T from one another.

With the divided recording pulse generating methods 1, 2 and 3 describedabove, by holding constant the switching period of at least intermediatepulse group (α_(i)+β_(i))T or (α_(i)+β_(i−1))T (2≦i≦m−1) at 1T, 1.5T, 2Tor 2.5T, and by changing the duty ratio of α_(i)−β_(i) and duty ratio ofα_(i)′−β_(i)′, it is possible to find an optimum divided recording pulsestrategy easily even when mediums with different characteristics areused or when the same medium is used at different linear velocities.

The optical recording method of this invention is particularly effectivefor a phase change medium in which information is overwritten by formingan amorphous mark on a crystal-state medium, the crystal state beingtaken as an unrecorded or erased state.

The optical recording method of this invention is also effective incases where the recording is made on the same medium at different linearvelocities. Generally, a constant density recording is commonlypracticed, which does not depend on the linear velocity but keeps aproduct of vT at a plurality of linear velocities constant, where v is alinear velocity and T is a clock period.

When for example the recording based on the mark length modulationscheme is to be performed on the same recording medium at a plurality oflinear velocities v in such a way that v×T is constant, the pulsegeneration method 2, for L equal to or more than 2, keeps the periods of(α_(i)+β_(i))T and (α_(i)′+β_(i)′)T for 2≦i≦m−1 constant irrespective ofthe linear velocity, also keeps Pw_(i), Pb_(i) and Pe for each i almostconstant irrespective of the linear velocity, and reduces α_(i) andα_(i)′ (1≦i≦m) as the linear velocity becomes slower (JP-A 9-7176). As aresult, a satisfactory overwrite is made possible in a wide range oflinear velocity.

When the recording based on the mark length modulation scheme is to beperformed on the same recording medium at a plurality of linearvelocities v with v×T kept constant, the pulse generation method 3, forL equal to or more than 2, keeps the periods of (β_(i−1)+α_(i))T and(β_(i−1)′+α_(i)′)T for 2≦i≦m constant irrespective of the linearvelocity, also keeps Pw_(i), Pb_(i) and Pe for each i almost constantirrespective of the linear velocity, and monotonously reduces α_(i) andα_(i)′ as the linear velocity becomes slower (JP-A 9-7176). In thiscase, too, a satisfactory overwrite is made possible in a wide range oflinear velocity.

In the above two examples, the expression “Pw_(i), Pb_(i) and Pe arealmost constant irrespective of the linear velocity” means that theminimum value is within about 20% of the maximum value, more preferablywithin 10%. Still more preferably, Pw_(i), Pb_(i) and Pe are virtuallyconstant, not dependent of the linear velocity at all.

In the above two examples, the method of reducing α_(i) and increasingβ_(i) in (α_(i)+β_(i))T and reducing α_(i) and increasing β_(i−1) in(α_(i)+β_(i))T as the linear velocity decreases is particularlyeffective in the phase change medium. This is because in the phasechange medium, the cooling speed of the recording layer becomes sloweras the linear velocity decreases and it is necessary to accelerate thecooling effect by increasing the ratio of the off pulse section β_(i).In that case, for all linear velocities v used and for all L, it ispreferred that β_(i) and β_(i)′ be set to 0.5<β_(i)≦2.5 and0.5≦β_(i)′≦2.5, more preferably 1≦β_(i)≦2 and 1≦β_(i)′≦2, to secure thecooling time to change the medium into the amorphous state.

In the above two examples, it is further preferred that, for all linearvelocities, α_(i)T and α_(i)′T (2≦i≦m−1) be held constant, i.e., theintermediate recording pulses have almost constant absolute lengths oftime. The expression “almost constant” means that they have a variationrange of about ±0.1T at each linear velocity. In that case, thereference clock T becomes large as the linear velocity decreases, soα_(i) and α_(i)′ in the intermediate pulse group necessarily decreasemonotonously. Although the first recording pulse sections α₁T, α₁′T canbe made constant, they should preferably be finely adjusted at eachlinear velocity. The β_(m) and β_(m)′ are preferably fine-adjusted ateach linear velocity. In that case, it is preferred that β_(m) andβ_(m)′ be set constant or made to increase as the linear velocitydecreases.

In the above two pulse generating methods 1, 2 and 3, when the referenceclock period T is smaller than the ×1-speed of the recordable DVD(linear velocity 3.5 m/s; and reference clock period T is 38.2nanoseconds), n−(η₁+η₂) and the first and last pulses should preferablybe controlled according to the preceding and/or subsequent mark lengthsor space lengths.

Examples in which the present invention proves particularly effectiveare described below.

A first case is where the linear velocity during the recording is set ashigh as 10 m/s or more and the shortest mark length as small as 0.8 μmor less in order to perform high density recording. Because the shortestmark length is expressed as nT×V where V is the linear velocity, thereduced shortest mark length results in the reference clock period Tbeing shortened.

It is also effective to set the wavelength of the recording light to asshort as 500 nm or less, the numerical aperture of the lens for focusingthe recording light to as high as 0.6 or more, the beam diameter of therecording light to a small value, and the shortest mark length to assmall as 0.3 μm or less to perform high density recording.

Further, it is also effective to use high density recording modulationscheme, such as a 8-16 modulation scheme and a (1, 7)-RLL-NRZImodulation scheme, as the mark length modulation scheme.

Another case is where the mark length modulation scheme is an EFMmodulation scheme and the linear velocity during recording is set to avery high speed of 10 times the CD reference linear velocity of 1.2 m/sto 1.4 m/s while keeping the recording line density constant during therecording.

Still another case is where the mark length modulation scheme is an EFM+modulation scheme, the high density recording scheme, and the linearvelocity during recording is set to as high as two or more times the DVDreference linear velocity of 3.49 m/s while keeping the recording linedensity constant during the recording.

Next, the quality of the mark length modulation signal will be describedby referring to the drawings.

FIG. 5 is a schematic diagram showing retrieved waveforms (eye-pattern)of the EFM modulation signal used in the CD family including Cd-RW. Inthe EFM modulation, the recording mark and space lengths can take a timelength of between 3T and 11T and the eye-pattern virtually randomlyincludes retrieved waveforms of all amorphous marks from 3T to 11T. TheEFM+ modulation further includes a mark length of 14T and a space lengthof 14T.

The upper end I_(top) of the eye-pattern converted into the reflectanceis an upper end value R_(top), and the amplitude of eye-pattern (inpractice, amplitude of 11T mark) I₁₁ standardized by the I_(top) is amodulation m₁₁ of the recording signal expressed as follows.

$\begin{matrix}{m_{11} = {\frac{I_{11}}{I_{top}} \times 100(\%)}} & (1)\end{matrix}$m₁₁ is preferably set between 40% and 80% and it is particularlyimportant to set m₁₁ to 40% or more. It is preferred that the signalamplitude be set large, but too large a signal amplitude will result inthe gain of the amplifier of the signal reproducing system becomingexcessively saturated. So, the upper limit of m₁₁ is set at around 80%.Too small a signal amplitude on the other hand will reduce thesignal-noise ratio (SN ratio) and thus the lower limit is set at around40%.

Further it is preferred that the asymmetry value Asym defined by theequation below be set as close to 0 as possible.

$\begin{matrix}{{Asym} = {\left( {\frac{I_{slice}}{I_{11}} - \frac{1}{2}} \right)(\%)}} & (2)\end{matrix}$

Further, it is desired that the jitter of each mark and space of theretrieved signal be almost 10% or less of the reference clock period Tand that the mark length and space length have nearly nT×V (T is areference clock period of data, n is an integer from 3 to 11, and v is alinear velocity during reproduction). With this arrangement, a signalreproduction using a commercially available CD-ROM drive can beperformed at a low error rate. In a recordable DVD medium using the EFM+modulation scheme, equations (1) and (2) are defined by replacing I₁₁with an amplitude I₁₄ of a 14T mark. The jitter is measured as aso-called edge-to-clock jitter, which is obtained by passing an analogretrieved signal through an equalizer to digitize it. In that case, thevalue of jitter is preferably 13% or less of the clock period,particularly 9% or less.

Next, a preferred optical recording medium for use in theabove-described optical recording method will be explained.

Optical recording mediums recorded according to this invention include apigment-based organic recording medium, a magnetooptical recordingmedium, a phase change recording medium and various other types ofrecording mediums. They also include a write-once and rewritablemediums. Of these mediums, the one that can produce a particularlysignificant effect is the phase change recording medium, particularly arewritable phase change recording medium in which an amorphous mark isoverwritten on a crystal-state medium, the crystal state being taken asan unrecorded state.

A particularly preferred material of the recording layer is of a type inwhich crystallization initiates at an interface between a crystal areaand a melted area.

Among the preferred phase change mediums are those having a recordinglayer containing still more excessive Sb in the SbTe eutecticcomposition. A particularly preferred composition is the one whichcontains excessive Sb and also Ge in the base Sb₇₀Te₃₀ eutecticcomposition. The Sb/Te ratio is particularly preferably set to 4 ormore. The content of Ge is preferably 10 atomic % or less. An example ofsuch a recording layer is a M_(z)Ge_(y)(Sb_(x)Te_(1-x))_(1-y-z) alloy(where 0≦z≦0.1, 0<y≦0.3, 0.8≦x; and M is at least one of In, Ga, Si, Sn,Pb, Pd, Pt, Zn, Au, Ag, Zr, Hf, V, Nb, Ta, Cr, Co, Mo, Mn, Bi, O, N andS).

The alloy with the above composition, as explained above, is a binaryalloy containing excessive Sb at the Sb₇₀Te₃₀ eutectic point and whichcontains Ge for improving the time-dependent stability and jitter, andalso contains at least one of the series of elements represented by Mfor further reduction of jitter and improvement of linear velocitydependency and optical characteristics. Alternatively, a compositionwith the Te amount close to zero can be regarded as an alloy that has Teor M element added in the composition near the Ge₁₅Sb₈₅ eutectic point.

In the above composition, Ge acts to enhance the time-dependentstability of the amorphous mark without degrading the high speedcrystallization function offered by excess Sb. It is considered to havea capability to raise the crystallization temperature and enhance theactivation energy for crystallization. That is, the above-mentionedalloy recording layer consisting mainly of GeSbTe in the base SbTeeutectic composition can increase the Sb/Te ratio while suppressing theformation of crystal nucleus by the presence of Ge and thereby increasethe speed of crystal growth. Generally, the forming of crystal nucleusinitiates at a lower temperature than that of the crystal growth andthis is not desirable to the storage stability of the mark at around theroom temperature when amorphous marks are formed. In the alloy recordinglayer with the above GeSbTe as a main component, because the crystalgrowth at near the melting point is selectively promoted, this alloy iscapable of quick erasure and has an excellent stability of the amorphousmark at room temperature. In this sense, the alloy recording layerdescribed above is particularly suited for high linear velocityrecording.

As the element M in the above composition, In and Ga may be used. Inparticular is effective in reducing jitter and enlarging the associatedlinear velocity margin. A more preferred composition of the recordinglayer of the phase change medium is A¹ _(a)A²_(b)Ge_(c)(Sb_(d)Te_(1-d))_(1-a-b-c) alloy (where 0≦a≦0.1, 0<b≦0.1,0.02<c≦0.3, 0.8≦d; A¹ is at least one of Zn, Pd, Pt, V, Nb, Ta, Cr, Co,Si, Sn, Pb_(i)Bi, N, O and S; and A² is In and/or Ga).

These compositions are preferable because, compared with the compositionnear the conventional GeTe—Sb₂Te₃ pseudo-binary alloy, the reflectanceof individual fine crystal grains has a smaller dependency on thedirection of crystal plane, providing these compositions with theability to reduce noise.

Further, the SbTe-based composition with the above Sb/Te ratio higherthan 80/20 is excellent in that it is capable of quick erasure at highlinear velocities equal to or more than 12 times the CD linear velocity(about 14 m/s) or 4 times the DVD linear velocity (about 14 m/s).

This composition, on the other hand, poses a particularly large problemwhen the reference clock period is as small as 25 ns or less. The reasonis described as follows.

The erasure of the amorphous mark in the above composition is virtuallygoverned only by the crystal growth from the boundary with the crystalarea surrounding the amorphous mark, and the formation of a crystalnucleus inside the amorphous mark and the process of crystal growth fromthe crystal nucleus hardly contribute to the recrystallization process.As the linear velocity increases (e.g., to more than 10 m/s), the timethat the erase power Pe is irradiated becomes short, extremely reducingthe time that the layer is kept at a high temperature around the meltingpoint necessary for the crystal growth. In the above composition,although the crystal growth from the area surrounding the amorphous markcan be promoted by increasing the Sb content, the increased content ofSb also increases the crystal growth speed during the re-solidifying ofthe melted area. That is, increasing the Sb content to ensure the quickerasure of the amorphous mark during the high linear velocity recordingmakes the formation of good amorphous marks difficult. In other words,when the speed of recrystallization from around the amorphous mark isincreased above a certain level, the recrystallization from around themelted area during the re-solidifying of the melted area formed torecord the amorphous mark is also accelerated.

In the composition described above, there is a problem that an attemptto perform erasure at high speed to effect a high linear velocityrecording makes the formation of an amorphous mark difficult. Inaddition, at a high linear velocity the clock period is shortened,reducing the off pulse section and degrading the cooling effect, whichin turn renders that problem even more conspicuous.

The composition problem described above is considered relatively not solarge with the commonly used conventional GeTe—Sb₂Te₃ pseudo-binaryalloy-based composition. In the GeTe—Sb₂Te₃ pseudo-binary alloy-basedcomposition, the erasure of the amorphous mark is effected mostly by theformation of crystal nuclei within the amorphous mark and not very muchby the crystal growth. Further, the formation of crystal nuclei is moreactive than the crystal growth at low temperatures. Hence, in theGeTe—Sb₂Te₃ pseudo-binary alloy-based composition, there-crystallization can be achieved by generating a large number ofcrystal nuclei even when the crystal growth is relatively slow. Further,during the process of re-solidification at temperatures below themelting point, the crystal nuclei are not generated and the speed ofcrystal growth is relatively small, so that the recording layer iseasily transformed into the amorphous state at a relatively smallcritical cooling speed.

The recording layer having a composition containing excess Sb in theSbTe eutectic composition, particularly a composition further includingGe, should preferably have a crystal state consisting of a virtuallysingle phase, not accompanied by phase separation. The crystal state canbe obtained by performing an initialization operation, which involvesheating and crystallizing the recording layer of amorphous stateproduced at an initial phase of the film deposition process usingsputtering. The expression “virtually single phase” means that therecording layer may be formed of a single crystal phase or a pluralityof crystal phases and that when it is formed of a plurality of crystalphases, it preferably has no lattice mismatch. When it is formed of asingle crystal phase, the recording layer may be multiple crystal layersof the same crystal phase but with different orientations.

The recording layer of such a virtually single phase can improvecharacteristics, such as reduced noise, an improved storage stabilityand a greater ease with which crystallization can be effected at highspeed. This may be explained as follows. When various crystal phases,including a crystal phase of a hexagonal structure, a cubic crystal suchas Sb but with a largely differing lattice constant, a face-centeredcubic crystal such as found in AgSbTe2, and other crystal phasesbelonging to other space groups, exist in a mixed state, a grainboundary with a large lattice mismatch is formed. This is considered tocause disturbances to the peripheral geometry of the mark and alsoproduce optical noise. In the recording layer of a single phase,however, such a grain boundary is not formed.

The type of crystal phase formed in the recording layer depends largelyon the initialization method performed on the recording layer. That is,to produce a preferred crystal phase in this invention, the recordinglayer initializing method should preferably incorporate the followingprovisions.

The recording layer is normally formed by a physical vacuum depositionsuch as sputtering. The as-deposited state immediately after the film isformed normally is an amorphous state and thus should be crystallized toassume an unrecorded/erased state. This operation is called aninitialization. The initialization operation includes, for example, anoven annealing in a solid phase in a temperature range from thecrystallization temperature (normally 150-300° C.) up to the meltingpoint, an annealing using light energy irradiation by a laser beam andlight of a flash lamp, and an initialization by melting. To obtain arecording layer of a preferred crystal state, the melting initializationis preferred. In the case of annealing in the solid phase there is atime margin for establishing a thermal equilibrium and thus othercrystal phases are likely to be formed.

In the melting crystallization, it is possible to melt the recordinglayer and then directly recrystallize it during the re-solidificationprocess. Or, it is possible to change the recording layer to theamorphous state during the re-solidification process and thenrecrystallize it in solid phase at near the melting point. In that case,when the crystallization speed is too slow, it may bring about a timemargin for the thermal equilibrium to be established thereby formingother crystal phases. Therefore it is preferred that the cooling speedbe increased to some extent.

For example, the time during which to hold the recording layer above themelting point is preferably set normally to 2 μs or less, morepreferably 1 μs or less. For the melting initialization a laser beam ispreferably used. It is particularly desirable for the initialization touse a laser beam which is elliptical with its minor axis oriented almostparallel in the direction of scan (this initialization method mayhereinafter be referred to as a “bulk erase”). In that case, the lengthof major axis is normally 10-1,000 μm and the minor axis normally 0.1-10μm. The lengths of major axis and minor axis of the beam are defined asa half width of the light energy intensity distribution measured withinthe beam. The scan speed is normally about 3-10 m/s. When the scanningis performed at speeds higher than the maximum usable linear velocity atwhich at least the phase change medium of this invention can beoverwrite-recorded, the area that was melted during the initializationscan may be transformed into the amorphous state. Further, scanning atspeeds 30% or more lower than the maximum usable linear velocitygenerally causes a phase separation, making it difficult to produce asingle phase. A scan speed 50-80% of the maximum usable linear velocityis particularly preferred. The maximum usable linear velocity itself isdetermined as the upper limit of a linear velocity that can assure acomplete erasure when the medium is irradiated with the Pe power at thatlinear velocity.

A laser beam source may use a semiconductor laser, a gas laser andothers. The power of the leaser beam is normally between approximately100 mW and 2 W.

During the initialization by the bulk erase, when a disklike recordingmedium is used, for example, it is possible to match the direction ofthe minor axis of the elliptical beam almost to the circumferentialdirection, scan the disk in the minor axis direction by rotating thedisk, and move the beam in the major axis (radial) direction for everyrevolution, thus initializing the whole surface. The distance moved bythe beam in the radial, direction for each revolution is preferably madeshorter than the beam major axis to overlap the scans so that the sameradius is irradiated with the laser beam a plurality of times. Thisarrangement allows for a reliable initialization and avoids an Uneveninitialized state that would be caused by the energy distribution(normally 10-20%) in the radial direction of the beam. When the distancetraveled is too small, other unwanted crystal phases are likely to beformed. Hence, the distance of travel in the radial direction isnormally set to ½ or more of the beam major axis.

The melting initialization may also be accomplished by using two laserbeams, melting the recording layer with a preceding beam, andrecrystallizing the recording layer with the second beam. If thedistance between the two beams is long, the area melted by the precedingbeam solidifies first before being recrystallized by the second beam.

Whether the melting/recrystallization has been performed or not can bedetermined by checking whether a reflectance R1 of the erased state,after the recording layer has been actually overwritten with anamorphous mark by the recording light about 1 μm across, is virtuallyequal to a reflectance R2 of the unrecorded state after initialization.When a signal pattern for recording amorphous marks intermittently isused, the measurement of R1 is carried out after a plurality ofoverwrites, normally approximately 5 to 100 overwrites, have beenperformed. This eliminates the influences of the reflectance of thespaces that could remain in the unrecorded state after one recordingoperation alone.

The above erased state may be obtained, rather than by modulating thefocused recording laser beam according to the actual recorded pulsegeneration method, but by irradiating the recording power DC-wise tomelt the recording layer and then resolidifying it.

In the case of the recording medium of this invention, the differencebetween R1 and R2 is preferably set as small as possible.

In more concrete terms, it is preferred that a value involving R1 and R2which is defined as follows be set 10(%) or less, particularly 5(%) orless.

$\frac{2{{{R1} - {R2}}}}{{R1} + {R2}} \times 100(\%)$

For example, in the phase change medium with R1 of around 17%, R2 needsto be in the range of 16-18%.

To realize such an initialized state, it is desired that almost the samethermal history as the actual recording condition be given by theinitialization.

The single crystal phase obtained by such an initialization methodgenerally tends to be a hexagonal crystal when the Sb/Te ratio is largerthan approximately 4.5 and a face-centered cubic crystal when the Sb/Teratio is less than 4.5. But this does not depend only on the Sb/Teratio. In the recording at speeds 16 times the CD linear velocity andfour times the DVD linear velocity, it is preferred that the recordinglayer be made of a single phase of hexagonal polycrystal.

The phase change medium of this invention normally has formed on thesubstrate a lower protective layer, a phase change recording layer, anupper protective layer and a reflection layer. It is particularlypreferred to form a so-called rapid cooling structure in which therecording layer is 10-30 nm thick, the upper protective layer is 15-50nm thick and the reflection layer is 30-300 nm thick. When the recordingmethod of this invention is to be applied to the above optical recordingmedium, n/m associated with the time lengths of all recording marksshould preferably be set to 1.5 or more. Further, n/m is more preferably1.8 or more. The upper limit of n/m normally is approximately 4,preferably approximately 3, but can change depending on other conditionssuch as the recording power Pw and the bias power Pb. Basically, n/mneeds only to fall in a range that gives a sufficient time length forcooling.

When the optical recording method is to be applied to a write-once typemedium, a setting should be made such that Pe=Pb=Pr (Pr is a retrievinglight power). It is also possible to set Pe>Pr to provide a residualheat effect.

The recording method of this invention does not depend on the layerstructure of the recording medium or the light radiating method, and canbe applied not only to a medium which has a layer structure ofsubstrate/protective layer/recording layer/protective layer/reflectionlayer and in which a retrieve/write laser beam is radiated through thesubstrate but also to a so-called film-side incident type medium whichhas a layer structure of substrate/reflection layer/protectivelayer/recording layer/protective layer and in which the retrieve/writelaser beam is radiated from the side opposite the substrate. Further,the recording method of this invention can also be applied to a mediumthat combines these mediums to form multiple recording layers.

The reflection layer has a function of promoting heat dissipation andenhancing the cooling speed. Hence, in the recording medium of thisinvention, the selection of the reflection layer is important.Specifically, it is preferred in this invention that a reflection layerused have a high heat dissipating effect.

The thermal conductivity of the reflection layer is considered to benearly inversely proportional to its volume resistivity and the heatdissipating effect of the reflection layer is proportional to the filmthickness. So, the heat dissipating effect of the reflection layer isconsidered generally to be inversely proportional to the sheetresistivity. In this invention, therefore, a reflection layer with asheet resistivity of 0.5 Ω/□ or less, particularly 0.4 Ω/□ or less, ispreferably used. The volume resistivity is preferably in the range ofbetween approximately 20 nΩ·m and 100 nΩ·m. A material with too small avolume resistivity is practically not usable. A material with too largea volume resistivity tends not only to have a poor heat dissipatingeffect but to degrade the recording sensitivity.

Possible materials for the reflection layer include aluminum, silver andalloys of these materials as main components.

Examples of aluminum alloy that can be used for the reflection layer arealuminum alloys having added to Al at least one of Ta, Ti, Co, Cr, Si,Sc, Hf, Pd, Pt, Mg, Zr, Mo and Mn. The contents of the additive elementsare normally between 0.2 atomic % and 1 atomic %. When these contentsare too small, hillock resistance tends to be insufficient; and whenthey are too large, the heat dissipating effect tends to deteriorate.

Examples of silver alloy that can be used for the reflection layer aresilver alloys having added to Ag at least one of Ti, V, Ta, Nb, W, Co,Cr, Si, Ge, Sn, Sc, Hf, Pd, Rh, Au, Pt, Mg, Zr, Mo and Mn. The additiveelements are preferably at least one of Ti, Mg, Pd and Cu metal elementsin terms of enhancing the time-dependent stability. The contents of theadditive elements are normally between 0.2 atomic % and 3 atomic %. Whenthese contents are too small, the corrosion resistance tends todeteriorate; and when they are too large, the heat dissipating effecttends to deteriorate.

The volume resistivity increases in proportion to the contents of theadded elements in the Al alloy and to the contents of the added elementsin the Ag alloy.

The reflection layer is normally formed by sputtering and vacuumdeposition methods. Because the total amount of impurities in thereflection layer, including water and oxygen trapped therein during thefilm making, should preferably be 2 atomic % or less, it is desired thatthe vacuum level in the process chamber used for forming the layer beset to 1×10⁻³ Pa or less. To reduce the amount of impurities trapped,the deposition rate is preferably set to 1 nm/sec or higher,particularly 10 nm/sec or higher. The amount of impurities trapped alsodepends on the method of manufacture of an alloy target used in thesputtering and on the sputter gas (rare gas such as Ar, Ne and Xe).

To enhance the heat dissipating effect of the reflection layer, thematerial of the reflection layer preferably consists of only aluminumand silver, as practically as possible.

The reflection layer may be formed in multiple layers to increase theheat dissipating effect and the reliability of the medium.

For example, when the reflection layer is made mainly of silver whichhas a large heat dissipating effect and a protective layer containingsulfur is provided between the reflection layer and the recording layer,the influences of silver and sulfur may pose problems with therepetitive overwrite characteristic and with a corrosion resistanceunder an accelerated test environment at high temperature and humidity.To avoid these problems an interface layer formed of an aluminum-basedalloy can be provided between these two layers so that a 2-layerreflection layer consisting of an aluminum layer and a silver layer canbe obtained. In that case, the thickness of the interface layer isnormally between approximately 5 nm and 100 nm, preferably between 5 nmand 50 nm. When the interface layer is too thin, the protective effectstends to be insufficient; and when it is too thick, the heat dissipatingeffect tends to deteriorate.

Forming the reflection layer in multiple layers is effective also forobtaining a desired sheet resistivity at a desired thickness of layer.

Now, the present invention will be explained in detail by taking exampleembodiments. It should be noted that the invention is not limited tothese embodiments but can be applied to whatever applications are withinthe spirit of the invention.

Embodiment 1

Over a polycarbonate substrate 1.2 mm thick formed with a trackinggroove (track pitch of 1.6 μm, groove width of about 0.53 μm, and groovedepth of about 37 nm), a (ZnS)₈₀(SiO₂)₂₀ protective layer was depositedto a thickness of 70 nm, a Ge₅Sb₇₇Te₁₈ recording layer to 17 nm, a(ZnS)₈₅(SiO₂)₁₅ protective layer to 40 nm, and an Al_(99.5)Ta_(0.5)alloy to 220 nm by sputtering in the vacuum chamber. An ultravioletcuring protective coat was applied over this substrate to a thickness of4 μm and cured to manufacture a phase change type rewritable opticaldisk.

This rewritable disk was subjected to the initial crystallizationprocess using a bulk eraser with a laser waveform of 810 nm and a beamdiameter of about 108 μm×1.5 μm at a power of 420 mW. Further in anevaluation apparatus having a laser wavelength of 780 nm and a pickupnumerical aperture NA of 0.55, the grooves and the lands werecrystallized once with a DC light of 9.5 mW by activating a servo toreduce noise of the crystal level.

Then, in the evaluation apparatus with a laser wavelength of 780 nm anda pickup numerical aperture NA of 0.55, the grooves were overwrittenwith an EFM modulation random pattern under the conditions: linearvelocity of 12 m/s (×10-speed of CD), base clock frequency of 43.1 MHz,and reference clock period T of 23.1 nanoseconds. The EFM modulationscheme uses marks having time lengths ranging from 3T to 11T. A patternin which these marks with different mark time lengths are randomlygenerated is an EFM modulation random pattern.

These patterns were overwrite-recorded by using the above-describedpulse division scheme 3 (the division number is set to m=1, 2, 2, 3, 3,4, 5, 5, 5 for n=3, 4, 5, 6, 7, 8, 9, 10, 11) with the recording powerPw set to 18 mW, the erase power Pe to 9 mW and the bias powerPb=retrieving power Pr to 0.8 mW. This pulse division scheme was able tobe realized by slightly changing the pulse generating circuit of FIG. 1.

Retrieving was done at a speed of 2.4 m/s (×2-speed of CD) and theretrieve signal was passed through a 2-kHz high frequency pass filterand then DC-sliced and retrieve by taking the center of the signalamplitude as a threshold value.

Before performing the overwrite, the pulse division scheme was optimizedin each of the mark time lengths ranging from 3T to 11T. Specifically,the first recording pulse section α₁T and the last off pulse sectionβ_(m)T were optimized.

An example case is shown in which an 11T mark (1.27 microseconds at×2-speed) was divided into five parts and the recording pulse widths andoff pulse widths were determined.

Using the pulse division scheme shown in FIG. 6(a), the pulse widthswere recorded by changing only α₁. The α₁-dependency of the retrievemark time length at the linear velocity of 2.4 m/s is shown in FIG. 7.For α₁=1.0, the mark time length was 1.28 microseconds, which was mostpreferable. The theoretical value is 1.27 microseconds.

Similarly, using the pulse division scheme shown in FIG. 6(b),measurements were made of the β_(m) (m=5) dependency. FIG. 8 shows theβ_(m)-dependency of the retrieve mark time length at the linear velocityof 2.4 m is equivalent to two times the CD linear velocity. Forβ_(m)=β₅=1.0, the mark time length was 1.35 microseconds.

These experiments were conducted on the marks having respective marktime lengths in order to optimize, in particular, the first recordingpulse α₁ and the last off pulse β₅. The pulse division scheme shown inFIG. 9 was determined. For the long marks with 8T to 11T lengths, α₁=1.0and β_(m)=1.0 were set.

After the optimization, the pulse division scheme of FIG. 9 was used tooverwrite the amorphous marks in the crystal area. The measurements ofthe mark time lengths of the retrieve signals for individual inputsignals of nT marks are shown in FIG. 10. The mark length change waslinear and the mark length deviation of the retrieve marks was in arange that allows the 3T-11T marks to be correctly distinguished anddetected. The jitter value here was low, well below the CD standard'sjitter upper limit of 17.5 nanoseconds for the ×2-speed reproduction,and the modulation was 0.6 or higher. This indicates that the recordingsignal thus obtained was satisfactory. In the figure, the mark lengthrefers to a mark time length and the space length refers to a space timelength.

Next, by using the pulse division scheme of FIG. 9, the EFM randomsignal was overwritten. The random signal was generated using AWG520manufactured by Sony Techtronix. At this time the pulse division wasoptimized for each mark length. As a result, even when the randomsignals were generated, desired mark lengths and satisfactory marklength jitter and space length jitter below 17.5 ns were obtained duringthe reproduction at ×2-speed.

When the random pattern was recorded, it was verified by a transmissionelectron microscope that the nT marks were not divided into a pluralityof amorphous portions but formed into a continuous amorphous mark.

Embodiment 2

Over a polycarbonate substrate 1.2 mm thick formed with a trackinggroove (track pitch of 1.6 μm, groove width of about 0.53 μm, and groovedepth of about 37 nm), a (ZnS)₈₀(SiO₂)₂₀ protective layer was depositedto a thickness of 70 nm, a Ge₇Sb₇₉Te₁₄ recording layer to 17 nm, a(ZnS)₈₅(SiO₂)₁₅ protective layer to 40 nm, and an Al_(99.5)Ta_(0.5)alloy to 220 nm by sputtering in the vacuum chamber. An ultravioletcuring protective coat was applied over this substrate to a thickness of4 μm and cured to manufacture an optical disk.

This rewritable disk was subjected to the initial crystallizationprocess using a bulk eraser with a laser waveform of 810 nm and a beamdiameter of about 108 μm×1.5 μm at a power of 420 mW. Further in anevaluation apparatus having a laser waveform of 780 nm and a pickupnumerical aperture NA of 0.55, the grooves and the lands werecrystallized once with a DC light of 9.5 mW by activating a servo toreduce noise of the crystal level.

Then, in the evaluation apparatus with a laser waveform of 780 nm and apickup numerical aperture NA of 0.55, the grooves were recorded withamorphous marks 11T in time length by using the pulse division schemeshown in FIG. 6(c) under the conditions: linear velocity of 19.2 m/s(×16-speed of CD), base clock frequency of 69.1 MHz, and reference clockperiod T of 14.5 nanoseconds.

The overwrite-recording was performed using the recording power Pw of 18mW, the erase power Pe of 9 mW and the bias power Pb=retrieving power Prof 0.8 mW.

The retrieving was performed at 2.4 m/s (×2-speed of CD) and theretrieved signal was passed through a 2-kHz high frequency pass filterand then DC-sliced and retrieve by taking the center of the signalamplitude as a threshold value.

The mark jitter was 13.1 nanoseconds and the space jitter 13.2nanoseconds, well below the CD standard's jitter upper limit of 17.5nanoseconds.

An EFM modulation random pattern was recorded and retrieved in a mannersimilar to the embodiment 1. The result was satisfactory.

Examples for Comparison 1

In the evaluation apparatus with a laser waveform of 780 nm and a pickupnumerical aperture NA of 0.55, the disk manufactured in the embodiment 2was recorded with amorphous marks 11T in time lengths and spaces 11T intime length alternately by using the n−k division scheme (m=n−k, n=1,the minimum of n/m is 1.1) of FIG. 11 currently employed in the CD-RW,under the conditions: linear velocity of 19.2 m/s (×16-speed of CD),base clock frequency of 69.1 MHz, and reference clock period T of 14.5nanoseconds.

The overwrite-recording was performed using the recording power Pw of 18mW, the erase power Pe of 9 mW and the bias power Pb=retrieving power Prof 0.8 mW.

When the signal was retrieved at the linear velocity of 2.4 m/s, thereflectance corresponding to a central portion of the mark of theretrieved signal did not fall sufficiently. Examination of the markfound that the central portion of the mark was significantlyrecrystallized. The jitter exceeded the 17.5-nanosecond limitsubstantially and was too high to be measured. To preventrecrystallization, the recording pulse widths were narrowed while stillin the n−1 division scheme but the modulation of the recording laserbeam could not follow the narrowed pulses, resulting in an increasedrecording power Pw and showing no improvements in the cooling effect.

Embodiment 3

Over a polycarbonate substrate 1.2 mm thick formed with a trackinggroove, which has a track pitch of 1.6 μm, a groove width of about 0.53μm and a groove depth of about 37 nm, a (ZnS)₈₀(SiO₂)₂₀ protective layerwas deposited to a thickness of 70 nm, a Ge₇Sb₇₈Te₁₅ recording layer to17 nm, a (ZnS)₈₀(SiO₂)₂₀ protective layer to 45 nm, and anAl_(99.5)Ta_(0.5) alloy reflection layer to 220 nm (volume resistivityof about 100 nΩ·m and sheet resistivity of 0.45 Ω/□) by sputtering inthe vacuum chamber. An ultraviolet curing resin protective coat wasapplied over this substrate to a thickness of 4 μm. A guide groove fortracking was given groove meanders 30 nm in amplitude (peak-to-peak)which were formed by frequency-modulating a 22.05-kHz carrier wave by ±1kHz. That is, address information was provided in the form of so-calledATIP along the spiral groove.

As in the embodiment 1 and 2, the disk was arranged so that a major axisof a focused laser beam was oriented in the direction of the diskradius, the laser beam having a wavelength of about 810 nm and anelliptical shape about 108 μm in major axis by about 1.5 μm in minoraxis. The disk was scanned at a linear velocity of 3-6 m/s andirradiated with a power of 400-600 mW for initialization. Further, inthe evaluation apparatus with a laser wavelength of 780 nm and a pickupnumerical aperture NA of 0.55, a servo was activated to crystallize thegrooves and the lands once with 9.5 mW of DC light to reduce the noiseof the crystallization level.

For the retrieve/write evaluation, a Pulsetech DDU1000 (wavelength of780 nm, NA=0.55) was used to write into and retrieve from the grooves.The retrieving was performed at ×2-speed irrespective of the linearvelocity used for recording. The jitter tolerance value for the CDformat in this case is 17.5 nanoseconds. As a signal source forgenerating gate signals, an arbitrary waveform signal source AWG520 ofSony Techtronix made was used.

First, the recording was made at a linear velocity 16 times the CDlinear velocity (19.2 m/s) and the reference clock period T was 14.5nanoseconds.

(1) First, the optimum condition for the intermediate pulse group wasstudied by using the divided recording pulses of FIG. 14. The recordingpower Pw_(i) was set constant at 20 mW, the bias power Pb_(i) was alsoset constant at 0.8 mW and the erase power Pe for spaces was set to 10mW.

As shown in FIG. 14(a), in the divided recording pulses having constantα_(i)=1, β_(i) was set to βc (constant value) and then changed toexamine the dependency of the amorphous mark formation on the off pulsesection length.

When the off pulse section was shorter than about 1T, the signalamplitude at the front end of the mark was low due to therecrystallization at the mark front end as shown in FIG. 3(d). At therear end, too, the amplitude was somewhat low. The maximum amplitude inthe entire mark length divided by the erase level signal intensity(×100%) was defined as a modulation, and the dependency of themodulation on the off pulse section is shown in FIG. 15(a). It is seenthat when the off pulse section was short, the modulation deteriorateddue to the influence of the waveform distortion (bad formation of theamorphous mark). When the off pulse section exceeded 1T, the modulationbecame saturated, producing a waveform close to a rectangular wavewithout distortion.

Next, using the divided recording pulses as shown in FIG. 14(b) with theoff pulse section set to 1.5T, the dependency of the modulation on therecording pulse section was examined. In FIG. 14(b), α_(i) was set to αc(a constant value) and changed uniformly. FIG. 15(b) shows theαc-dependency of modulation. It is seen that a nearly saturatedmodulation was obtained for αc=1 to 1.5.

(2) Next, the divided recording pulses of FIG. 16 with the intermediatepulse group fixed to α_(i)=1 and β_(i)=1.5 were used and the control ofthe mark length and the characteristic of the mark end was examined bycontrolling the first period and the last period. In FIG. 16, one 0.5Trecording pulse section was added at the rear end of the mark to makethe mark length close to 11T accurately. This made both of the marklength and the space length assume 11T and the condition for obtaining asatisfactory jitter was searched. The original waveform was a repetitivepattern of the 11T mark and the 11T space, with the first recordingpulse rising in synchronism with the front end of the 11T mark. Here,because the ×2-speed retrieving was performed, the upper limit of thejitter allowable value was 17.5 nanoseconds (ns) and the 11T wasequivalent to about 1.27 microseconds (μs). FIGS. 17, 18 and 19 showthese values with dotted lines.

Using the divided recording pulses as shown in FIG. 16(a), thedependency on the front recording pulse α₁ length was checked. FIGS.17(a) and 17(b) represent the α₁-dependency of the mark length and spacelength and the α₁-dependency of the mark jitter and the space jitter,respectively. It is seen from FIG. 17(b) that α₁ is preferably set to0.8-1.8 to keep the jitter below 17.5 nanoseconds.

In FIG. 17(b) the desired 11T was not obtained for the mark length andspace length. So, α₁ was set to α₁=1, and the divided recording pulsesas shown in FIG. 16(b) were used to examine the dependency on the firstoff pulse β₁T length. FIGS. 18(a) and 18(b) represent the β₁-dependencyof the mark length and the space length and the β₁-dependency of themark jitter and the space jitter, respectively. It is seen that almostthe desired mark length and space length were obtained for β₁=1.3 andthat satisfactory jitters were obtained for a β₁ range of between 1 and1.7. Here, β₁=1.5 was chosen.

Further, using the divided recording pulses as shown in FIG. 16(c) andsetting α₁=1 and β₁=1.5, the dependency on the last off pulse β_(m)length was studied. FIGS. 19(a) and 19(b) show the β_(m)-dependency ofthe mark length and the space length and the β_(m)-dependency of themark jitter and the space jitter, respectively. The figures show thatthe desired mark length and space length were obtained for β_(m)=around0.7 and that satisfactory jitters were obtained in a wide range ofβ_(m)=0 to 1.8.

These show that setting α₁=1, β₁=1.5 and β_(m)=0.8 results in thedesired 11T mark length and minimum jitters.

(3) With the results of the above (1) and (2) taken into account, apulse dividing method based on the (divided recording pulse generatingmethod 2) described above and using a base period of 2T was performed,in a range of α₁=1±0.5 and β₁=1±0.5, on the EFM modulation signal whichconsists of 3T to 11T mark lengths. The specific pulse dividing methodfor each mark length is shown in FIG. 20.

That is, for the mark recording in which n is even, i.e., the marklength is nT=2LT, where L is an integer equal to or more than 2, themark is divided into m=L sections and the recording pulse section α_(i)where the recording power Pw_(i)is to be radiated and the off pulsesection β_(i) where the bias power Pb_(i) is to be radiated are set asfollows:α₁+β₁=2α_(i)+β_(i)=2(2≦i≦m−1)α_(m)+β_(m)=1.6

For the mark recording in which n is odd, i.e., the mark length isnT=(2L+1)T, the mark is divided into m=L sections and each pulsesections is set as follows:α₁′+β₁′=2.5α_(i)′+β_(i)′=2(2≦i≦m−1)α_(m)′+β_(m)′=2.1Although the division number is the same m=L for the 2LT mark and the(2L+1)T mark, the first period and the last period are differentiatedbetween these marks by giving them a 0.5T difference.

In FIG. 20 the delay of α₁T from the front end of the nT mark is set toT_(d1)=0. For n≧4, the intermediate pulse group is held constant atα_(i)=0.8 and β_(i)=1.2 (2≦i≦m−1) irrespective of the n value.

Further, when n is even, the following settings are made: α₁=0.8,β₁=1.2, α_(m)=0.7 and β_(m)=0.9. When n is odd, the following settingsare made: α₁′=1.0, β₁′=1.5, α_(m)′=1.0 and β_(m)′=1.1. Only the 3T casewas exceptional. A 3T mark length was obtained for α₁=1.2 and β₁=1.5. InFIG. 20, the recording pulse section and the off pulse section arerepresented by the top and bottom portions of the rectangular wave.Individual lengths of sections are indicated by numbers, and thedepicted lengths of the top and bottom portions in the figure are notscaled to the exact lengths of the sections.

The recording power Pw_(i) and the bias power Pb_(i) were set constantirrespective of the i value, i.e., Pw=20 mW and Pb=0.8 mW. the erasepower Pe was set to 10 mW.

After 9 overwrites were performed (the initial recording was deemed a0-th recording), measurements were made of the mark length and spacelength and also jitters for each nT mark and nT space. The measurementsof mark lengths and space lengths are shown in FIG. 21(a) and themeasurements of jitters of the marks and spaces are shown in FIG. 21(b).The mark lengths and space lengths were almost precisely nT and thejitters were below 17.5 nanoseconds although the jitters degraded 2-3nanoseconds from the initial recording due to overwriting. Instead ofperforming overwrite, the erase power Pe was radiated DC-wise for eraseoperation. This resulted in a jitter improvement of about 2 nanoseconds.

-   -   (4) An overwrite was performed on the same medium at ×10-speed        of CD by changing the clock period so that the product of the        linear velocity v and the clock period T was constant. That is,        the reference clock period T in this case was 23.1 nanoseconds.        For n≧4, α_(i)T (1≦i≦m) was held almost constant. That is, the        intermediate recording pulse group was held constant at        α_(i)=0.5 and β_(i)=1.5 (2≦i≦m−1).

The divided pulses are as shown in FIG. 22. When n was even, pulses wereset to α₁=0.6, β₁=1.4, α_(m)=0.5 and βm=1.4. When n was odd, pulses wereset to α_(i)′=0.6, β_(i)′=1.9, α_(m)′=0.6 and β_(m)′=1.8. Only the 3Tcase was exceptional. A 3T mark length was obtained for α₁=0.8 andβ₁=2.4. This divided recording pulses correspond, except for n=3,roughly to multiplying the clock period by 16/10 (inversely proportionalto the linear velocity) while holding the recording pulse lengthobtained in FIG. 20 constant. The recording power Pw_(i) and the biaspower Pb_(i) were held constant at Pw=20 mW and Pb=0.8 mW irrespectiveof the i value as in the case with the ×16-speed. The erase power Pe wasalso set to 10 mW as in the case with the ×16-speed.

After 9 overwrites were performed (the initial recording was deemed a0-th recording), measurements were made of the mark length and spacelength and also jitters for each nT mark and nT space. The measurementsof mark lengths and space lengths are shown in FIG. 23(a) and themeasurements of jitters of the marks and spaces are shown in FIG. 23(b). The mark lengths and space lengths were almost precisely nT and thejitters were below 17.5 nanoseconds although the jitters degraded 2-3nanoseconds from the initial recording due to overwriting.

Instead of performing overwrite, the erase power Pe was radiated DC-wisefor erase operation. This resulted in a jitter improvement of about 2nanoseconds.

(5) An overwrite was performed on the same medium by using a repetitivepattern (11T pattern) consisting of 11T mark with divided recordingpulses and 11T spaces, and a repetitive pattern (3T pattern) consistingof 3T mark with divided recording pulses and 3T spaces. Afteroverwriting the 3T pattern nine times, the 11T pattern was overwrittenat the 10th time and a rate of reduction in the carrier level of the 3Tsignal (in unit of dB) was measured as an erase ratio (overwrite eraseratio). Although the 3T pattern was slightly deviated among differentlinear velocities, both the 3T and 11T patterns were basically changedaccording to the division method of FIG. 20 so that α_(i)T (1≦i≦m)remained almost constant.

The erase ratio was evaluated by changing the linear velocity whilekeeping the product of the linear velocity and the reference clockperiod constant. The overwrite erase ratio of 20 dB or more was obtainedfor the 10, 12, 16 and 18 times the CD linear velocity.

When a random pattern was recorded, it was verified with a transmissionelectron microscope that the nT marks were not divided into a pluralityof amorphous portions but formed into a continuous amorphous mark.

The recording layer similar to that used above was peeled off afterbeing initialized and its crystallinity was observed with a transmissionelectron microscope. The observation found that the recording layer wasa polycrystal formed of a single phase of hexagonal crystal. The crystalphase was found to have no phase separation and is assumed to have asingle phase polycrystalline structure with the orientations rotated. Anexamination using an X-ray diffraction found that it had a hexagonalstructure.

Embodiment 4

Over a polycarbonate substrate 0.6 mm thick formed with a trackinggroove, which has a track pitch of 0.74 μm, a groove width of about 0.27μm and a groove depth of about 30 nm, a (ZnS)₈₀(SiO₂)₂₀ protective layerwas deposited to a thickness of 68 nm, a Ge₅Sb₇₇Te₁₈ recording layer to14 nm, a (ZnS)₈₀(SiO₂)₂₀ protective layer to 25 nm, and anAl_(99.5)Ta_(0.5) alloy reflection layer to 200 nm (volume resistivityof about 100 nΩ·m and sheet resistivity of 0.5 Ω/□) by sputtering in thevacuum chamber. An ultraviolet curing resin layer was applied over thissubstrate to a thickness of 4 μm by a spin coat. This is bonded withanother substrate 0.6 mm thick having the same structure of layers toform a phase change disk.

As in the embodiment 3, the disk thus obtained was arranged so that amajor axis of a focused laser beam was oriented in the direction of thedisk radius, the laser beam having a wavelength of about 810 nm and anelliptical shape about 108 μm in major axis by about 1.5 μm in minoraxis. The disk was scanned at a linear velocity of 3-6 m/s andirradiated with a power of 400-600 mW for initialization. Further, inthe evaluation apparatus with a laser wavelength of 660 nm and a pickupnumerical aperture NA of 0.65, tracking and focus servos were activatedto scan about 6 mW of DC light over the grooves once at 4 m/s to reducethe noise of the crystallization level.

For the retrieve/write evaluation, a Pulsetec DDU1000 (wavelength ofabout 660 nm, NA=0.55) was used to write into and retrieve from thegrooves. As a signal source for generating gate signals, an arbitrarywaveform signal source AWG610 manufactured by Sony Techtronix was used.In this case, the length of a 3T mark was 0.4 μm and the clock period ateach linear velocity was so set that the recording density would be thesame as that of DVD (26.16 MHz at 3.5 m/s).

First, the linear velocity during the recording was set to 16.8 m/s(clock frequency of 125.93 MHz and clock period of 7.9 nsec) equivalentto the ×4.8-speed of DVD; a 14T section was divided by using simplewaveforms as shown in FIG. 25; and the intermediate divided recordingpulses were examined. The space was set to 14T. The recording power wasset to a constant value of Pw=15 mW, the erase power to Pe=5 mW, and thebias power to Pb=0.5 mW. The recording power application section wasdenoted Tw and the bias power application section Tb. Two cases werestudied: in the first case Tw+Tb=1T was set and Pw and Pb were appliedfor 14T periods (FIG. 25(a)); and in the second case Tw+Tb=2T was setand Pw and Pb were applied for 7T periods (FIG. 25(b)). In each of thesetwo cases, the dependency of the modulation of the recording markportion of the retrieved signal on a ratio of Tw to T (Tw/T) wasevaluated. When the Tw/T for 2T periods was 1.0, the signal obtained wasa square wave almost free of distortion and the modulation was maximum.When the Tw/T ratio was less than 0.5, the waveform was distorted. Thisis considered due to the insufficient recording power applicationsection and therefore an insufficient temperature rise. Conversely, whenthe Tw/T is more than 1.0, as Tw increases, the modulation decreases.This is considered due to the insufficient cooling time, which preventsthe transformation to the amorphous state by recrystallization. WhenTw/T exceeds 1.5, the modulation falls below 5%, resulting in adistorted waveform (not shown). For 1T period, the modulation was lowover the entire range and only the distorted waveforms were produced.This is because in the 1T period there might not be a range where therecording power application time and the cooling time were bothsufficient.

It can be seen from the foregoing discussion that, in the dividedrecording pulse generating method 2 or 3, the intermediate dividedrecording pulse group for at least 2≦i≦m−1 is preferably set toα_(i)=α_(i)′=1 and β_(i)=β_(i)′=1.

Next, it was verified as follows that the disk discussed above wascapable of high speed erasure at high linear velocities of 14 m/s and17.5 m/s (equivalent to 4 and 5 times the DVD linear velocity of 3.5m/s). That is, the overwrite was performed by using a repetitive pattern(8T pattern) consisting of 8T mark with divided recording pulses and 8Tspaces, and a repetitive pattern (3T pattern) consisting of 3T mark withdivided recording pulses and 3T spaces. After overwriting the 3T pattern9 times, the 8T, pattern was overwritten at the 10th time and a rate ofreduction in the carrier level of the 3T signal was determined as anoverwrite erase ratio. The overwrite erase ratio was determined bykeeping the product of the linear velocity and the reference clockperiod constant so that the same recording density as the DVD wasobtained. The overwrite erase ratio of 25 dB or more was obtained for 14m/s and 17.5 m/s.

Further, a pulse dividing method based on the divided recording pulsegenerating method 3 described above and using a base period of 2T, wasperformed on a EFM+ modulation signal consisting of 3T-11T and 14T,marks. This EFM+ modulation signal was recorded at 14 m/s and 16.8 m/s(3 and 4.8 times the DVD linear velocity of 3.5 m/s). For the ×4-speed,the clock frequency was 104.9 MHz and the clock period was 9.5 nsec. Forthe ×4.8-speed, the clock frequency was 125.9 MHz and the clock periodwas 7.9 nsec. The specific pulse dividing method is as shown in FIG. 26.

For the mark recording in which n is even, i.e., the mark length isnT=2LT (L is an integer equal to or more than 2), the mark is dividedinto m=L sections and α_(i) and β_(i) in the recording pulse sectionα_(i)T and the off pulse section β_(i)T are set as follows:T_(d1)+α₁=2(T_(d1)=0.95)β_(i−1)+α_(i)=2(2≦i≦m−1)

For the mark recording in which n is odd, i.e., the mark length isnT=(2L+1)T, the mark is divided into m=L sections and α_(i) and β_(i) inthe recording pulse section α_(i)T and the off pulse section β_(i)T areset as follows:T_(d1)′+α₁′=2.05(T_(d1)′=1)β₁′+α₂′=2.45β_(i−1)′+α_(i)′=2(3≦i≦m−1)β_(m−1)′+α_(m)′=2.45In this case, for L=2, β₁′+α₂′=2.9 and α_(m)=1 and α_(m)′=α_(m)+0.2=1.2.

In the case of L≧3, the intermediate recording pulse group was set toconstant values: α_(i)′=α_(i)=1 and β_(i)′=β_(i)=1 (2≦i≦m−1), andα_(m)=α_(m)′=1. For L≧2, they were set to constant values, not dependenton the n value: α₁=α₁′=1.05 and β_(m)=β_(m)′=0.4.

Further, in the case of 3T, a 3T mark length was obtained withT_(d1)=1.15, α₁=1.2 and β₁=0.8. In FIG. 26, the recording pulse sectionand the off pulse section are represented by the top and bottom portionsof the rectangular wave. Specific lengths of sections are indicated bynumbers, and the depicted lengths of the top and bottom portions in thefigure do not correspond to the lengths of the sections.

The bias power Pb_(i) was set to a fixed value Pb=0.5 mW, not dependenton the i value, and the erase power Pe was set to 4.5 mW. The recordingpower Pw_(i) was also set to a fixed value irrespective of the i value.After overwriting 9 times, the edge-to-clock jitter and the dependencyof the modulation on the recording power were measured. Retrieving wasperformed using the reproducing light power of Pr=0.8 mW and the linearvelocity of 3.5 m/s. At either recording linear velocity and with therecording power of 15.0 mW, the edge-to-clock jitter was less than 10%and the modulation achieved 60% or higher, as shown in FIGS. 27(a) and27(b). R_(top) was about 18%. Measurement of the overwrite dependency atthe recording power of 15.0 mW found that, as shown in FIG. 27(c), theedge-to-clock jitter was 11% or less even after 10,000 overwriteoperations. At this time R_(top) and the modulation exhibited almost nochange with the overwrite.

Further, a pulse dividing method of FIG. 28 based on the dividedrecording pulse generating method 3 described above was performed on thesimilar disk by recording an EFM+ modulation signal at a linear velocityof 7 m/s, equivalent to two times the DVD linear velocity, and a clockfrequency of 52.5 MHz (clock period of 19.1 nsec).

As in the case with 4 and 4.8 times the DVD speed, the bias power wasset constant at Pb=0.5 mW and the erase power Pe at 4.5 mW. Therecording power Pw_(i) was also set constant, not dependent on the ivalue. After nine overwrite operations, the edge-to-clock jitter and therecording power dependency of the modulation were measured. As shown inFIGS. 27(a) and 27(b), at the recording power of 13.0 mW, theedge-to-clock jitter was less than 8% and the modulation achieved 57% orhigher. R_(top) was about 18%. At the recording power of 13.0 mW, theoverwrite dependency was measured and it was found that, as shown inFIG. 27(c), the edge-to-clock jitter was below 11% even after 10,000overwrite operations. At this time R_(top) and the modulation exhibitedalmost no change with the overwrite.

From the above discussion, it is understood that the use of the pulsedividing method based on the divided recording pulse generation method 3enables recording in a linear velocity range of 2 to 4.8 times the DVDlinear velocity. Hence, with this method the recording with a constantangular velocity can be performed in a radial range, for example, fromabout 24 mm to about 58 mm, which constitutes a data area of DVD.

INDUSTRIAL APPLICABILITY

According to this invention, even when the reference clock period isshort, a satisfactory mark length modulation recording can be performed,allowing a higher density and a faster recording of the opticalrecording media. This in turn leads to an increase in the recordablecapacity of the optical disk and enables the recording speed andtransfer rate of the optical disk to be enhanced, greatly expanding therange of its applications for recording large amounts of data such asmusic and video and for external storage devices of computers. Forinstance, it is possible to realize a rewritable CD that overwrites EFMmodulation marks at speeds more than 12 times the CD linear velocity anda rewritable DVD that overwrites EFM+ modulation marks at speeds morethan 4 times the DVD linear velocity.

1. An optical recording method for recording mark length-modulatedinformation with a plurality of recording mark lengths by irradiating arecording medium with a light, the optical recording method comprisingthe steps of: when a time length of one recording mark is denoted nT (Tis a reference clock period equal to or less than 25 ns, and n is anatural number equal to or more than 2), dividing the time length of therecording mark nT intoη₁T, α₁T, β₁T, α₂T, β₂T, . . . , α_(i)T, β_(i)T, . . . , α_(m)T, β_(m)T,η₂T  in that order (m is a pulse division number; ΣhdiΣ_(i)(α_(i)+β_(i))+η₁+η₂=n; α_(i) (1≦i≦m−1) (1≦i≦m) is a real numberlarger than 0; βi β_(i) (1≦i≦m−1) is a real number larger than 0; β_(m)is a real number larger than or equal to 0; α_(i)+β_(i) (2≦i≦m−1) orβ_(i−)1 β_(i−l)+α_(i) (2≦i≦m−1) is kept constant independently of saidreal number i; and η₁ and η₂ are real numbers between −2 and 2);radiating recording light with a recording power Pw_(i), in a timeduration of α_(i)T (1≦i≦m); and radiating recording light with a biaspower Pb_(i) in a time duration of β_(i)T(1≦i≦m−1), the bias power beingPb_(i)<Pw_(i) and Pb_(i)<Pw_(i+1); wherein the pulse division number mis 2 or more for the time duration of at least one recording mark andmeets n/m≧1.25 for the time length of all the recording marks, furtherwherein when the same pulse division number m is used on at least tworecording marks with different n values, a difference mark length isformed by changing at least one of β₁, β_(m−1), and βm β_(m).
 2. Anoptical recording method according to claim 1, wherein α_(i)+β_(i)(2≦i≦m−1) or β_(i−)1 β_(i−l)+α_(i) (2≦i≦m−1) is 2 independently of saidreal number i.
 3. An optical recording method according to claim 1,wherein α_(i) is kept constant as a constant value αc where said i is(2≦i≦m−1).
 4. An optical recording method according to claim 1, whereinα_(i)(2≦i≦m−1) is kept constant in the time length of the recording markwith having a pulse division number m being at least
 3. 5. An opticalrecording method according to claim 1, wherein when performing a marklength modulation scheme recording on the same recording medium by usinga plurality of linear velocities v while keeping v×T constant, for mequal to or greater than 2, (α_(i)+β_(i)) in 2≦i≦m−1 is kept constantindependently of the linear velocity, Pw₁ PW_(i), Pb_(i) and Pe in eachi are kept almost constant independently of the linear velocity, andα_(i) (2≦i≦m) is decreased as the linear velocity lowers.
 6. An opticalrecording method according to claim 1, wherein when performing a marklength modulation scheme recording on the same recording medium by usinga plurality of linear velocities v while keeping v×T constant, for mequal to or greater than 2, (β_(i−)1 β_(i−l)+α_(i)) in 2≦i≦m are keptconstant independently of the linear velocity, Pw₁ PW_(i), Pb₁ and Pe ineach i are kept almost constant independently of the linear velocity,and α_(i) (2≦i≦m) are decreased as the linear velocity lowers.
 7. Anoptical recording method according to claim 5 or 6, wherein α_(i)T(2≦i≦m−1) is kept almost constant independently of the linear velocity.8. An optical recording method according to claim 1, the phase changetype optical recording medium having a recording layer made ofM_(z)Ge_(y)(Sb_(x)Te_(1-x))_(1-y-z) alloy (where 0≦_(z)≦0.1, 0<_(y)≦0.3,0.8≦x; and M is at least one of In, Ga, Si, Sn, Pb Pd, Pt, Zn, Au, Ag,Zr, Hf, V, Nb, Ta, Cr, Co, Mo, Mn, Bi, O, N and S).
 9. An Anon-transitory optical information recording medium having a recordinglayer, containing excessive Sb in SbTe eutectic point, in which phasechange is made reciprocally between a crystal state and amorphous statewith optical characteristic being differed from each other byirradiation of an optical beam, wherein said crystal condition isdefined as polycrystal made of a substantial single crystal phase of ahexagonal crystal.
 10. An optical information recording medium accordingto claim 9, wherein said recording layer is made ofM_(z)Ge_(y)(Sb_(x)Te_(1-x))_(1-y-z) alloy (where 0≦z≦0.1 0<y≦0.3, 0.8≦x;and M is at least any one of In, Ga, Si, Sn, Pb, Pd, Pt, Zn, Au, Ag, Zr,Hf, V, Nb, Ta, Cr, Co, Mo, Mn, Bi, O, N and S).
 11. An opticalinformation recording medium according to claim 9 or 10, wherein saidcrystal state of said recording layer is defined as an unrecorded stateand an erased state, while said amorphous state thereof is defined as arecorded state and an erased state, while said amorphous state thereofis defined as a recorded state so as to performed perform recording orerasing of information.
 12. A method of manufacturing an opticalinformation recording medium having a recording layer, containingexcessive Sb in SbTe eutectic point, in which phase change is maderecriprocally between a crystal state and amorphous state with opticalcharacteristic being differed from each other by irradiation of anoptical beam, wherein an initialization step is performed with anotheroptical beam having an elliptical beam shape of which minor axis is0.1-10 μm after forming at least said recording layer on a substrate, byscanning said another optical beam to the recording layer in a directionof said minor axis so as to make the recording layer in the crystalstate, further wherein said scanning of said optical beam is performedin a speed in a range of 50-80% of a maximum usable linear velocity forover-writing of the recording layer.